Astronomy

Angle of the Sun during winter solstice

Angle of the Sun during winter solstice

What is the angle of the Sun on the eastern horizon at 35° latitude if it is the winter solstice?


This Is What REALLY Happens To The Sun During The Winter Solstice

The winter solstice in the Northern Hemisphere is the shortest day of the year — and it's happening tomorrow, Dec. 21 (although the date could be different, depending on your time zone). In the U.S. and other parts of the hemisphere, the winter solstice also signifies the first official day of winter. Even though most of us know of the solstice, many are still unaware of what it means from a scientific standpoint. What does the sun do on the winter solstice? Let's explore.


Winter-Solstice Shadow Ratio & Azimuth Table

Determine objects’ shadow lengths and direction on the day with the longest shadows of the year (the winter solstice) using the following table of shadow ratios and sun’s azimuth angles for your latitude.

Use the ratio 1 : x to get the length of the shadow cast by an object (tree, cistern, house) for each foot (or meter) of height. Multiply the height of the object (in feet or meters) by x to get the length of the shadow (in the same units of measurement).

To get the direction of the shadow cast by an object, look at the Azimuth angle—the angle of the sun’s location east or west of True South in the northern hemisphere or of True North in the southern hemisphere. Note that when the position of the sun is to the east, it will cast a shadow to the west of any object. (See figures below the table for examples, and the “Integrated Design Pattern Five: Maintaining Winter Sun Exposure” section of Rainwater Harvesting for Drylands and Beyond, Volume 1, 2nd Edition, for still more examples, explanation, information, and tools.)

For more examples, explanation, and information on how to use the this table and other passive solar design tools, see Integrated Design Pattern Five: Maintaining Winter Sun Exposure section of Rainwater Harvesting for Drylands and Beyond, Volume 1, 2nd Edition, by Brad Lancaster.


STEMvisions Blog

I f you have ever gone swimming in summer or had a snowball fight in winter, then you know something about seasons . Seasons are times on Earth that have very specific weather patterns and hours of daylight. Earth’s four seasons are spring, summer, fall, and winter. Seasons are caused by Earth’s changing position as it revolves around the Sun. Some people think that the seasons occur because of Earth’s distance from the Sun. However, Earth is closer to the Sun in December and January and farther away from the Sun in July and August. So what’s going on?

All of the planets and bodies in our solar system orbit the Sun. This is called a heliocentric (sun-centered) orbit . Almost all of these objects also travel in the orbital plane . Think of this as a flat disk, with the Sun at the center, extending out toward the edges of our solar system.

As Earth travels in its orbital plane, it is held in place by the Sun’s gravitational field. Earth also tilts on its axis at 23.5 degrees to this orbital plane. This means that the north and south poles of our planet are not straight up and down as we orbit the Sun they are always at an angle. Earth’s rotation on its axis takes approximately 24 hours, which is why we have day and night. It takes Earth about 365.25 days to complete one entire revolution around the Sun, which gives us 1 year. As Earth revolves around the Sun, it rotates on its axis. Sometimes Earth tilts toward the Sun, which is when summer occurs. In the winter, Earth tilts away from the Sun.

The angle of the Sun above the horizon is much greater in summer than in winter. Image: Smithsonian Science Education Center

Summer Solstice

In North America, around June 21, Earth tilts on its axis toward the Sun. This is called the summer solstice, and it is when the Northern Hemisphere has the most daylight of any time of the year. It is also when the Sun appears to be the highest in the sky, and it has its greatest angle to Earth. The higher the Sun is, the more sunlight and heat Earth receives. This is when the days are longer and the nights are shorter.

The seasons in Earth’s Northern Hemisphere are always opposite those in the Southern Hemisphere. This means that at the northern summer solstice, North America will experience summer, and winter will start in in the Southern Hemisphere.

Winter Solstice

Around December 21, the Northern Hemisphere tilts the farthest away from the Sun. This is called the northern winter solstice, and it is when we have the least amount of daylight of any time of the year. During this time, the Sun appears lowest in the sky, with the least angle to Earth, and we receive less sunlight and warmth. This is also when the days are shorter and the nights are longer.

Between March 21 and September 21, the Northern Hemisphere is tilted toward the Sun and has spring and summer. During that same time, the Southern Hemisphere is tilted away from the Sun and has fall and winter. The equator is warm all year round. Image: Smithsonian Science Education Center

Around March 21 and September 21, Earth is angled approximately 90 degrees away from the Sun (a right angle). This is when the spring and fall equinoxes occur. During this time, both the Northern and Southern Hemispheres are exactly the same angle from the Sun. Therefore, we all experience about the same amount of daylight and darkness.

Image: HannamariaH/iStock/Thinkstock

Having four seasons is typical for people that live above and below Earth’s equator. People that live at the equator do not experience four seasons. This is because the middle of Earth does not tilt much as Earth rotates on its axis. Without tilt, the angle of the Sun is always the same, so the equator receives the same amount of warmth and light year round. The farther you get from the equator, the bigger the difference there is in seasons and sunlight. For example, in Barrow, Alaska, the northernmost city in the United States, they experience 4 months of complete sunlight from May to August and complete darkness from November to January.

This is an excerpt from the Space Systems Exploration unit of our curriculum product line, STCMS TM . Please visit our publisher, Carolina Biological, to learn more.


Contents

The English word winter comes from the Proto-Germanic noun *wintru-, whose origin is unclear. Several proposals exist, a commonly mentioned one connecting it to the Proto-Indo-European root *wed- 'water' or a nasal infix variant *wend-. [1]

The tilt of the Earth's axis relative to its orbital plane plays a large role in the formation of weather. The Earth is tilted at an angle of 23.44° to the plane of its orbit, causing different latitudes to directly face the Sun as the Earth moves through its orbit. This variation brings about seasons. When it is winter in the Northern Hemisphere, the Southern Hemisphere faces the Sun more directly and thus experiences warmer temperatures than the Northern Hemisphere. Conversely, winter in the Southern Hemisphere occurs when the Northern Hemisphere is tilted more toward the Sun. From the perspective of an observer on the Earth, the winter Sun has a lower maximum altitude in the sky than the summer Sun.

During winter in either hemisphere, the lower altitude of the Sun causes the sunlight to hit the Earth at an oblique angle. Thus a lower amount of solar radiation strikes the Earth per unit of surface area. Furthermore, the light must travel a longer distance through the atmosphere, allowing the atmosphere to dissipate more heat. Compared with these effects, the effect of the changes in the distance of the Earth from the Sun (due to the Earth's elliptical orbit) is negligible.

The manifestation of the meteorological winter (freezing temperatures) in the northerly snow–prone latitudes is highly variable depending on elevation, position versus marine winds and the amount of precipitation. For instance, within Canada (a country of cold winters), Winnipeg on the Great Plains, a long way from the ocean, has a January high of −11.3 °C (11.7 °F) and a low of −21.4 °C (−6.5 °F). [2] In comparison, Vancouver on the west coast with a marine influence from moderating Pacific winds has a January low of 1.4 °C (34.5 °F) with days well above freezing at 6.9 °C (44.4 °F). [3] Both places are at 49°N latitude, and in the same western half of the continent. A similar but less extreme effect is found in Europe: in spite of their northerly latitude, the British Isles have not a single non-mountain weather station with a below-freezing mean January temperature. [4]

Meteorological reckoning is the method of measuring the winter season used by meteorologists based on "sensible weather patterns" for record keeping purposes, [5] so the start of meteorological winter varies with latitude. [6] Winter is often defined by meteorologists to be the three calendar months with the lowest average temperatures. This corresponds to the months of December, January and February in the Northern Hemisphere, and June, July and August in the Southern Hemisphere. The coldest average temperatures of the season are typically experienced in January or February in the Northern Hemisphere and in June, July or August in the Southern Hemisphere. Nighttime predominates in the winter season, and in some regions winter has the highest rate of precipitation as well as prolonged dampness because of permanent snow cover or high precipitation rates coupled with low temperatures, precluding evaporation. Blizzards often develop and cause many transportation delays. Diamond dust, also known as ice needles or ice crystals, forms at temperatures approaching −40 °C (−40 °F) due to air with slightly higher moisture from above mixing with colder, surface-based air. [7] They are made of simple hexagonal ice crystals. [8] The Swedish meteorological institute (SMHI) defines thermal winter as when the daily mean temperatures are below 0 °C (32 °F) for five consecutive days. [9] According to the SMHI, winter in Scandinavia is more pronounced when Atlantic low-pressure systems take more southerly and northerly routes, leaving the path open for high-pressure systems to come in and cold temperatures to occur. As a result, the coldest January on record in Stockholm, in 1987, was also the sunniest. [10] [11]

Accumulations of snow and ice are commonly associated with winter in the Northern Hemisphere, due to the large land masses there. In the Southern Hemisphere, the more maritime climate and the relative lack of land south of 40°S makes the winters milder thus, snow and ice are less common in inhabited regions of the Southern Hemisphere. In this region, snow occurs every year in elevated regions such as the Andes, the Great Dividing Range in Australia, and the mountains of New Zealand, and also occurs in the southerly Patagonia region of South Argentina. Snow occurs year-round in Antarctica.

In the Northern Hemisphere, some authorities define the period of winter based on astronomical fixed points (i.e. based solely on the position of the Earth in its orbit around the Sun), regardless of weather conditions. In one version of this definition, winter begins at the winter solstice and ends at the March equinox. [12] These dates are somewhat later than those used to define the beginning and end of the meteorological winter – usually considered to span the entirety of December, January, and February in the Northern Hemisphere and June, July, and August in the Southern. [12]

Astronomically, the winter solstice, being the day of the year which has fewest hours of daylight, ought to be in the middle of the season, [13] [14] but seasonal lag means that the coldest period normally follows the solstice by a few weeks. In some cultures, the season is regarded as beginning at the solstice and ending on the following equinox [15] [16] – in the Northern Hemisphere, depending on the year, this corresponds to the period between 20, 21 or 22 December and 19, 20 or 21 March. [12]

In the United Kingdom, meteorologists consider winter to be the three coldest months of December, January and February. [17] In Scandinavia, winter in one tradition begins on 14 October and ends on the last day of February. [18] In Russia, calendar winter is widely regarded to start on 1 December [19] and end on 28 February. [20] In many countries in the Southern Hemisphere, including Australia, [21] [22] New Zealand, [23] and South Africa, winter begins on 1 June and ends on 31 August. In Celtic nations such as Ireland (using the Irish calendar) and in Scandinavia, the winter solstice is traditionally considered as midwinter, with the winter season beginning 1 November, on All Hallows, or Samhain. Winter ends and spring begins on Imbolc, or Candlemas, which is 1 or 2 February. This system of seasons is based on the length of days exclusively. (The three-month period of the shortest days and weakest solar radiation occurs during November, December and January in the Northern Hemisphere and May, June and July in the Southern Hemisphere.)

Also, many mainland European countries tended to recognize Martinmas or St. Martin's Day (11 November), as the first calendar day of winter. [24] The day falls at the midpoint between the old Julian equinox and solstice dates. Also, Valentine's Day (14 February) is recognized by some countries as heralding the first rites of spring, such as flowers blooming. [25]

In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, with the Jiéqì (known as 立冬 lì dōng – literally, "establishment of winter").

The three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere and lasts through late February or early March. This "thermological winter" is earlier than the solstice delimited definition, but later than the daylight (Celtic) definition. Depending on seasonal lag, this period will vary between climatic regions.

Cultural influences such as Christmas creep may have led to the winter season being perceived as beginning earlier in recent years, although high latitude countries like Canada are usually well into their real winters before the December solstice.

Since by almost all definitions valid for the Northern Hemisphere, winter spans 31 December and 1 January, the season is split across years, just like summer in the Southern Hemisphere. Each calendar year includes parts of two winters. This causes ambiguity in associating a winter with a particular year, e.g. "Winter 2018". Solutions for this problem include naming both years, e.g. "Winter 18/19", or settling on the year the season starts in or on the year most of its days belong to, which is the later year for most definitions.

Ecological reckoning of winter differs from calendar-based by avoiding the use of fixed dates. It is one of six seasons recognized by most ecologists who customarily use the term hibernal for this period of the year (the other ecological seasons being prevernal, vernal, estival, serotinal, and autumnal). [26] The hibernal season coincides with the main period of biological dormancy each year whose dates vary according to local and regional climates in temperate zones of the Earth. The appearance of flowering plants like the crocus can mark the change from ecological winter to the prevernal season as early as late January in mild temperate climates.

To survive the harshness of winter, many animals have developed different behavioral and morphological adaptations for overwintering:

    is a common effect of winter upon animals, notably birds. However, the majority of birds do not migrate – the cardinal and European robin, for example. Some butterflies also migrate seasonally. is a state of reduced metabolic activity during the winter. Some animals "sleep" during winter and only come out when the warm weather returns e.g., gophers, frogs, snakes, and bats.
  • Some animals store food for the winter and live on it instead of hibernating completely. This is the case for squirrels, beavers, skunks, badgers, and raccoons.
  • Resistance is observed when an animal endures winter but changes in ways such as color and musculature. The color of the fur or plumage changes to white (in order to be confused with snow) and thus retains its cryptic coloration year-round. Examples are the rock ptarmigan, Arctic fox, weasel, white-tailed jackrabbit, and mountain hare.
  • Some fur-coated mammals grow a heavier coat during the winter this improves the heat-retention qualities of the fur. The coat is then shed following the winter season to allow better cooling. The heavier coat in winter made it a favorite season for trappers, who sought more profitable skins.
  • Snow also affects the ways animals behave many take advantage of the insulating properties of snow by burrowing in it. Mice and voles typically live under the snow layer.

Some annual plants never survive the winter. Other annual plants require winter cold to complete their life cycle this is known as vernalization. As for perennials, many small ones profit from the insulating effects of snow by being buried in it. Larger plants, particularly deciduous trees, usually let their upper part go dormant, but their roots are still protected by the snow layer. Few plants bloom in the winter, one exception being the flowering plum, which flowers in time for Chinese New Year. The process by which plants become acclimated to cold weather is called hardening.

  • 1683–1684, "The Great Frost", when the Thames, hosting the River Thames frost fairs, was frozen all the way up to the London Bridge and remained frozen for about two months. Ice was about 27 cm (11 in) thick in London and about 120 cm (47 in) thick in Somerset. The sea froze up to 2 miles (3.2 km) out around the coast of the southern North Sea, causing severe problems for shipping and preventing use of many harbours.
  • 1739–1740, one of the most severe winters in the UK on record. The Thames remained frozen over for about 8 weeks. The Irish famine of 1740–1741 claimed the lives of at least 300,000 people. [27]
  • 1816 was the Year Without a Summer in the Northern Hemisphere. The unusual coolness of the winter of 1815–1816 and of the following summer was primarily due to the eruption of Mount Tambora in Indonesia, in April 1815. There were secondary effects from an unknown eruption or eruptions around 1810, and several smaller eruptions around the world between 1812 and 1814. The cumulative effects were worldwide but were especially strong in the Eastern United States, Atlantic Canada, and Northern Europe. Frost formed in May in New England, killing many newly planted crops, and the summer never recovered. Snow fell in New York and Maine in June, and ice formed in lakes and rivers in July and August. In the UK, snow drifts remained on hills until late July, and the Thames froze in September. Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in food shortages and the worst famine of the 19th century.
  • 1887–1888, there were record cold temperatures in the Upper Midwest, heavy snowfalls worldwide, and amazing storms, including the Schoolhouse Blizzard of 1888 (in the Midwest in January), and the Great Blizzard of 1888 (in the Eastern US and Canada in March).
  • In Europe, the winters of early 1947, [28] February 1956, 1962–1963, 1981–1982 and 2009–2010 were abnormally cold. The UK winter of 1946–1947 started out relatively normal, but became one of the snowiest UK winters to date, with nearly continuous snowfall from late January until March.
  • In South America, the winter of 1975 was one of the strongest, with record snow occurring at 25°S in cities of low altitude, with the registration of −17 °C (1.4 °F) in some parts of southern Brazil.
  • In the eastern United States and Canada, the winter of 2013–2014 and the second half of February 2015 were abnormally cold. However, the winter of 2014–2015 did have a balmy December and a normal January.
  • 1310–1330, many severe winters and cold, wet summers in Europe – the first clear manifestation of the unpredictable weather of the Little Ice Age that lasted for several centuries (from about 1300 to 1900). The persistently cold, wet weather caused great hardship, was primarily responsible for the Great Famine of 1315–1317, and strongly contributed to the weakened immunity and malnutrition leading up to the Black Death (1348–1350).
  • 1600–1602, extremely cold winters in Switzerland and Baltic region after eruption of Huaynaputina in Peru in 1600.
  • 1607–1608, in North America, ice persisted on Lake Superior until June. Londoners held their first frost fair on the frozen-over River Thames.
  • 1622, in Turkey, the Golden Horn and southern section of Bosphorus froze over.
  • 1690s, extremely cold, snowy, severe winters. Ice surrounded Iceland for miles in every direction.
  • 1779–1780, Scotland's coldest winter on record, and ice surrounded Iceland in every direction (like in the 1690s). In the United States, a record five-week cold spell bottomed out at −20 °F (−29 °C) at Hartford, Connecticut, and −16 °F (−27 °C) in New York City. Hudson River and New York's harbor froze over.
  • 1783–1786, the Thames partially froze, and snow remained on the ground for months. In February 1784, the North Carolina was frozen in Chesapeake Bay.
  • 1794–1795, severe winter, with the coldest January in the UK and lowest temperature ever recorded in London: −21 °C (−6 °F) on 25 January. The cold began on Christmas Eve and lasted until late March, with a few temporary warm-ups. The Severn and Thames froze, and frost fairs started up again. The French army tried to invade the Netherlands over its frozen rivers, while the Dutch fleet was stuck in its harbor. The winter had easterlies (from Siberia) as its dominant feature.
  • 1813–1814, severe cold, last freeze-over of Thames, and last frost fair. (Removal of old London Bridge and changes to river's banks made freeze-overs less likely.)
  • 1883–1888, colder temperatures worldwide, including an unbroken string of abnormally cold and brutal winters in the Upper Midwest, related to the explosion of Krakatoa in August 1883. There was snow recorded in the UK as early as October and as late as July during this time period.
  • 1976–1977, one of the coldest winters in the US in decades.
  • 1985, Arctic outbreak in US resulting from shift in polar vortex, with many cold temperature records broken.
  • 2002–2003 was an unusually cold winter in the Northern and Eastern US.
  • 2010–2011, persistent bitter cold in the entire eastern half of the US from December onward, with few or no mid-winter warm-ups, and with cool conditions continuing into spring. La Niña and negative Arctic oscillation were strong factors. Heavy and persistent precipitation contributed to almost constant snow cover in the Northeastern US which finally receded in early May.
  • 2011 was one of the coldest on record in New Zealand with sea level snow falling in Wellington in July for the first time in 35 years and a much heavier snowstorm for 3 days in a row in August.

Humans are sensitive to cold, see hypothermia. Snowblindness, norovirus, seasonal depression. Slipping on black ice and falling icicles are other health concerns associated with cold and snowy weather. In the Northern Hemisphere, it is not unusual for homeless people to die from hypothermia in the winter.

One of the most common diseases associated with winter is influenza. [29]

In Persian culture, the winter solstice is called Yaldā (meaning: birth) and it has been celebrated for thousands of years. It is referred to as the eve of the birth of Mithra, who symbolised light, goodness and strength on earth.

In Greek mythology, Hades kidnapped Persephone to be his wife. Zeus ordered Hades to return her to Demeter, the goddess of the Earth and her mother. However, Hades tricked Persephone into eating the food of the dead, so Zeus decreed that Persephone would spend six months with Demeter and six months with Hades. During the time her daughter is with Hades, Demeter became depressed and caused winter.

In Welsh mythology, Gwyn ap Nudd abducted a maiden named Creiddylad. On May Day, her lover, Gwythr ap Greidawl, fought Gwyn to win her back. The battle between them represented the contest between summer and winter.


How to Calculate the Winter Solstice Sun Angle

During a solstice, which occurs around Dec. 21 and June 21 every year, the Earth's axis is positioned relative to the sun such that one hemisphere is closest to the sun and the other is farthest from the sun. The hemisphere farthest from the sun experiences the winter solstice, with the direct rays of the sun falling 23.5 degrees north of the equator. Calculate the sun angle during the winter solstice for your location by determining your latitude and doing two simple calculations.

Consult an atlas or a geographical website to find the latitude of your location on Earth. For example, if you live in Cape Canaveral, Fla., your latitude is 28° 24' 21" N, or approximately 28.4 degrees.

Add 23.5 degrees to your latitude to compensate for the fact that the sun's direct rays fall on one of the tropic lines during the winter solstice: the Tropic of Cancer for the northern hemisphere and the Tropic of Capricorn for the southern hemisphere. For example, if you live in Cape Canaveral, add 23.5 to 28.4 to get 51.9 degrees.

Subtract this value from 90 degrees to get the angle of elevation from the horizon of the sun during midday on the winter solstice. In the above example, subtract 51.9 from 90 to get 39.1 degrees. This is the angle of elevation of the sun in Cape Canaveral at midday.


Boyne Valley Private Day Tours

Pick up and return to your accommodation or cruise ship. Suggested day tour: Newgrange World Heritage site, 10th century High Crosses at Monasterboice, Hill of Tara the seat of the High Kings of Ireland and the Hill of Slane where St. Patrick let a Paschal fire in 433 More .



Winter Solstice at Newgrange

Dark retreats before
the calculated caress
of sun's brightness.

Winter’s hand pulls back
from a small ancient chamber
light intensifies.

For a few minutes,
brilliance scatters kisses
before light recedes.

The night must return,
and we can draw light into
dark times if we try.


Alan Betson has created a selection of his best Winter Solstice photographs which are now available to purchase.


Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2019 September 23
Equinox: The Sun from Solstice to Solstice
Image Credit & Copyright: Tunç Tezel (TWAN)

Explanation: Today is an equinox, a date when day and night are equal. Tomorrow, and every day until the next equinox, the night will be longer than the day in Earth's northern hemisphere, and the day will be longer than the night in Earth's southern hemisphere. An equinox occurs midway between the two solstices, when the days and nights are the least equal. The featured picture is a composite of hourly images taken of the Sun above Bursa, Turkey on key days from solstice to equinox to solstice. The bottom Sun band was taken during the north's winter solstice in 2007 December, when the Sun could not rise very high in the sky nor stay above the horizon very long. This lack of Sun caused winter. The top Sun band was taken during the northern summer solstice in 2008 June, when the Sun rose highest in the sky and stayed above the horizon for more than 12 hours. This abundance of Sun caused summer. The middle band was taken during an equinox in 2008 March, but it is the same sun band that Earthlings see today, the day of the most recent equinox.


The Summer and Winter Solstices

You know them as the first days of winter and summer. Others refer to them as the shortest and longest days of the year. So, what makes these days &mdash the solstices &mdash special? To understand, you'll need a little background about the Sun and the Earth.

In the summer, days feel longer because the Sun rises earlier in the morning and sets later at night. When the North Pole of the Earth is tilted toward the Sun, we in the northern hemisphere receive more sunlight and it's summer. As the Earth moves in its orbit, the tilt of the North Pole changes (see diagram). When it is tilted away from the Sun, it is winter in the northern hemisphere. In between we have autumn and spring.

The day that the Earth's North Pole is tilted closest to the sun is called the summer solstice. This is the longest day (most daylight hours) of the year for people living in the northern hemisphere. It is also the day that the Sun reaches its highest point in the sky.

The winter solstice, or the shortest day of the year, happens when the Earth's North Pole is tilted farthest from the Sun.

In between, there are two times when the tilt of the Earth is zero, meaning that the tilt is neither away from the Sun nor toward the Sun. These are the vernal equinox &mdash the first day of spring &mdash and the autumnal equinox &ndash the first day of fall. Equinox means "equal." During these times, the hours of daylight and night are equal. Both are 12 hours long. Check out the chart below to see how the solstice works.


There are two solstice days each year corresponding to the longest day (the summer solstice) and shortest day (the winter solstice). The days of these events depend on the hemisphere:

Southern Hemisphere Northern Hemisphere
Summer Solstice 21 December 21 June
Winter Solstice 21 June 21 December

The exact date of each solstice changes by a few days each year – this is largely a consequence of our calendar system where we count years of 365 or 366 days, but the Earth takes 365.256 days (the sidereal period) to complete one orbit of the Sun. The exact orbital and daily rotational motion of the Earth, such as the ‘wobble’ in the Earth’s axis (precession), also contributes to the changing solstice dates.

The solstices occur because the rotation axis of the Earth is tilted by an angle of 23.5 degrees from the vertical. If the Earth’s rotation was at right angles to the plane of its orbit around the Sun, there would be no solstice days and no seasons. Around 21 June, the Sun is at its most northerly declination (+23.5 degrees). This corresponds to the northern summer solstice and marks the longest day of the year for northern hemisphere observers. In contrast, this is the date of the southern winter solstice and marks the shortest day of the year for southern hemisphere observers. Six months later, the Sun is at its most southerly declination (-23.5 degrees) and the solstices are reversed in each hemisphere.

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Angle of the Sun during winter solstice - Astronomy

As the Earth travels around the Sun in its orbit, the north-south position of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes with respect to the Sun. This QuickTime movie illustrates the tilt of the Earth's equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.

In the northern hemisphere, the Winter solstice is day of the year (near December 22) when the Sun is farthest south. However, in the southern hemisphere, winter and summer solstices are exchanged so that the winter solstice is the day on which the Sun is farthest north. The winter solstice marks the first day of the season of winter. The declination of the Sun on the (northern) winter solstice is known as the tropic of capricorn (-23° 27').

The winter solstice is the shortest day of the year, respectively, in the sense that the length of time elapsed between sunrise and sunset on this day is a minimum for the year. Of course, daylight saving time means that the first Sunday in April has 23 hours and the last Sunday in October has 25 hours, but these human meddlings with the calendar and do not correspond to the actual number of daylight hours. In Chicago, there are 9:20 hours of daylight on the winter solstice of December 22, 1999.

The above plots show how the date of the winter solstice shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the winter solstice. To convert to U. S. Eastern Standard Time, subtract 5 hours, so the winter solstice occurs on December 21, 1998 at 20:43 (8:43 p.m.) EST December 22, 1999 at 02:32 a.m. EST and December 21, 2000 at 08:23 a.m. EST.

Note that the times below were calculated using WinterSolstice [] in the Mathematica application package Scientific Astronomer , which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory (which computes December 22, 1999 at 07:44 UT instead of 07:32 UT and December 21, 2000 at 13:37 UT instead of 13:23).

Date UT Date UT Date UT
12-21-1980 16:52 12-22-1990 03:07 12-21-2000 13:23
12-21-1981 22:41 12-22-1991 08:56 12-21-2001 19:12
12-22-1982 04:31 12-21-1992 14:47 12-22-2002 01:01
12-22-1983 10:20 12-21-1993 20:36 12-22-2003 06:51
12-21-1984 16:10 12-22-1994 02:25 12-21-2004 12:41
12-21-1985 22:00 12-22-1995 08:14 12-21-2005 18:30
12-22-1986 03:49 12-21-1996 14:05 12-22-2006 00:20
12-22-1987 09:38 12-21-1997 19:54 12-22-2007 06:09
12-21-1988 15:29 12-22-1998 01:43 12-21-2008 11:59
12-21-1989 21:18 12-22-1999 07:32 12-21-2009 17:49

United States Government Printing Office. The Astronomical Almanac for the Year 1999. Washington, DC: Navy Dept., Naval Observatory, Nautical Almanac Office, p. A1, 1999.

United States Government Printing Office. The Astronomical Almanac for the Year 2000. Washington, DC: Navy Dept., Naval Observatory, Nautical Almanac Office, p. A1, 2000.


Watch the video: Why the Sun gets higher in Summer than it does in Winter. (September 2021).