I have heard, that on the northern hemisphere, you can estimate a direction of south if you imagine a line going through the tips of the border between illuminated and shadowed part of the Moon, and where this line crosses the horizon, that is estimated direction of the south (and vice versa from the southern hemisphere).
This inspired me to think about how much information could one get simply by observing the Moon. I think you can estimate the latitude of your location. I live in the northern hemisphere and never have been anywhere else, so it's all just an imagination. That's why I need somebody to confirm or correct my conclusions.
I think the angle of the tilt of the Moon will be different as seen from different latitudes. Specificaly near the poles you would see the line between illuminated and shadowed part of the Moon very verticaly. At the equator horizontaly. And somewhere in the middle between the equator and the pole tilted under some angle.
You would also see the Moon much lower on the horizon when standing near the poles and going near the zenith when standing on equator.
I also think you can use observation of the phase changes througout a month to find out whether you are at the southern or the northern hemisphere.
So this is what I imagine the Moon looks like. I have searched the internet for relevant information, but nobody seems to discuss the whole thing in detail. There is only one picture somewhat depicting the tilt of the Moon as seen from 3 different latitudes of the Earth and that's it.
EDIT A software for simulation of the skies (Stellarium) have been recommended to me by @barrycarter and after few hours playing with it, I can clonclude, that the graphics I provided above are accurate.
An interesting observation made while seeing the Moon during daytime. In the real life at the surface of the Earth this observation would not be possible, but the software allows it. Since the Sun is no longer positioned "under horizon", it illuminates the Moon from different angle, than depicted in the grapics.
I made few screeenshots from view from the France (northern hemisphere) of this situation to illustrate what I mean.
It is possible to see, that at nighttime, the moon looks exactly as in the graphics, but as soon as the Sun rises, the illumination direction changes.
Here are some answers to your various comments and questions:
you can estimate a direction of south
True, but it would be a very rough approximation at best. In order for it to be accurate, the bright limb of the Moon would need to face either due east or west on the celestial sphere. (In astronomy terminology, the position angle of the bright limb would need to be 90 or 270 degrees.) In reality, the bright limb of the Moon faces the Sun, which can be north or south of due east/west relative to the Moon. Therefore, the cusps are not north/south, so the cusps can point to the "left or right" of due south on the horizon.
The second problem is using two points just 1/2 degree apart (the two cusps) and extending it to where it meets the horizon (along a great circle, too!). Depending on your location and the Moon's declination, that distance is most likely to be much larger than 1/2 degree. At 40 degrees N latitude, the moon can be approximately 27 to 73 degrees from the southern horizon. (These numbers are when the Moon is on the meridian.) The accuracy of extending such a short line over such a long distance of is going to be very low.
I think you can estimate the latitude of your location
Technically, you can use any astronomical object to determine your latitude. Depending on how you do that, the Moon is probably the worse object to use. In order to estimate your latitude, you need to know the declination of the object (as well as other data or measurements). Because the Moon orbits the Earth, the declination changes "quickly". The Sun or stars are better because their declination moves more slowly or are fixed, respectively. Polaris is the easiest object to use if you are in the northern hemisphere.
I also think you can use observation of the phase changes throughout a month to find out whether you are at the southern or the northern hemisphere.
An interesting observation made while seeing the Moon during daytime. In the real life at the surface of the Earth this observation would not be possible, but the software allows it. Since the Sun is no longer positioned "under horizon", it illuminates the Moon from different angle, than depicted in the graphics.
The Moon is visible during the day, so I do not understand the second sentence. For the rest of the comment, I suggest that you choose a day when the Moon is closer to the Sun and run the simulations. For example, choose a day when the Moon is within 3 or 4 days of New Moon, and watch the Sun and Moon as they cross the sky. The bright limb of the Moon always faces toward the Sun (along a great circle), so the angle of illumination on the Moon changes throughout the day -- just like Stellarium shows. (The Moon's image for Oct 2017 at 8 hours looks a little off to me, but it may just be the difficulty of estimating the great circle direction from the image. Displaying the lines of right ascension and declination may help.)
Can the Moon be upside down?
Did you know that the Moon looks different from Earth’s northern and southern hemispheres? Someone looking at the Moon from our north pole would see it upside down compared to someone seeing it from the south pole. And someone on the equator would see it at various orientations throughout the day.
This fact is as fascinating as it is mind-bending, so let’s try to wrap our heads around it.
Because the Moon orbits the Earth around (though not directly in line with) the Earth’s equator, when you are on either of the Earth’s poles the Moon will never appear overhead rather, it will always be relatively close to the horizon and you'll perceive the side closest to the ground as the bottom. Because a person standing on Earth’s north pole is upside down compared to someone on the south pole, their perspective of the Moon would be upside down as well.
Different views of the Moon From Earth's northern hemisphere, the Moon's north pole appears at the top. From Earth's southern hemisphere, the Moon's south pole appears at the top. It's all a matter of perspective!
This difference is pretty easy to intuitively understand. The complicated part is what happens when you move closer to the equator.
It’s important to note here that the Moon doesn’t orbit the Earth perfectly in line with our equator. There are a few things going on here: the Earth’s axis is tilted, the Moon’s orbit is tilted, and everything is always moving and changing. But for simplicity’s sake, let’s pretend that the Moon orbits the Earth around the equator. This will make it easier to understand without changing any major conclusions about how we see the Moon from different latitudes.
In this case, if you were standing somewhere along the Earth’s equator, as the Earth rotated the Moon would rise in the east, pass directly overhead, and then set in the west. From this perspective, as the Moon rose it would have a top and bottom relative to the horizon like it does from any perspective on Earth. But after passing directly overhead, the Moon would set on the opposite side of the sky from where it rose. At that point, it would have the opposite orientation compared to the horizon. So if you asked a person who lived on the equator which part of the Moon was the top, they would have to ask you “at what time?”
Another strange thing about seeing the Moon from this perspective is that its orientation when it rose and set would both look sideways compared to what you’d see nearer either of the poles.
The Moon seen from its orbital plane On the left, the full Moon as seen from the equator at moonset, on the western horizon. On the right, the full Moon as seen from the equator at moonrise, on the eastern horizon.
If you traveled north from this middle point, you’d start to see the Moon rise and set farther south in the sky. Then, when you looked at the Moon, there would be an obvious top and bottom that would be consistent over the course of its apparent crossing of the sky.
Likewise, if you go south from the equator, you’ll see the Moon rise and set in the northern sky. Again, you’ll see an obvious top and bottom to the Moon. But it will be the opposite orientation from how you saw it when you were far north from its orbital plane.
The Moon seen from the poles On the left, the full Moon as it would be seen from the north pole. On the right, the full Moon as it would be seen from the south pole.
The difference between these orientations happens gradually. When you travel just a short distance from the equator, the Moon will still rise more or less due east and set more or less due west, and in between will appear almost directly overhead. But as you travel farther north or south, it will start to appear farther south or north in the sky, respectively.
This is all a bit tricky to wrap your head around, especially when you take into account the slight changes that Earth’s tilted axis creates, plus all the various moving parts: Earth rotating, the Moon orbiting the Earth, and the Earth orbiting the Sun. It’s a great reminder that even something as seemingly simple as our Moon is part of a complex and dynamic system.
This phenomenon happens with other celestial objects as well, as long as they lie near Earth’s orbital plane (which the Moon lies near, though not on). Jupiter, for instance, can look upside down from the north pole compared to how it looks from the south pole. What’s more, its stripes look horizontal when seen near the horizon in both the northern and southern hemispheres of Earth, but from the equator a rising Jupiter’s stripes are vertical.
Moon Features You Can See From Earth
What can you see on the Moon tonight? This guide from The Planetary Society will help you identify some features.
Earth and Moon as Seen from Mars
The High Resolution Imaging Science Experiment (HiRISE) camera would make a great backyard telescope for viewing Mars, and we can also use it at Mars to view other planets. This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter.
At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel, an Earth diameter of about 90 pixels and a moon diameter of 24 pixels. The phase angle is 98 degrees, which means that less than half of the disk of the Earth and the disk of the moon have direct illumination. We could image Earth and moon at full disk illumination only when they are on the opposite side of the sun from Mars, but then the range would be much greater and the image would show less detail.
On the day this image was taken, the Japanese Kayuga (Selene) spacecraft was en route from the Earth to the moon, and has since returned spectacular images and movies (see http://www.jaxa.jp/projects/sat/selene/index_e.html).
On the Earth image we can make out the west coast outline of South America at lower right, although the clouds are the dominant features. These clouds are so bright, compared with the moon, that they are saturated in the HiRISE images. In fact the red-filter image was almost completely saturated, the Blue-Green image had significant saturation, and the brightest clouds were saturated in the infrared image. This color image required a fair amount of processing to make a nice-looking release. The moon image is unsaturated but brightened relative to Earth for this composite. The lunar images are useful for calibration of the camera.
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft. The High Resolution Imaging Science Experiment is operated by the University of Arizona, Tucson, and the instrument was built by Ball Aerospace and Technology Corp., Boulder, Colo.
Huge and opaque ‘something’ dims a giant star
A computer model of VVV-WIT-08, showing the star, a mystery disk passing in front of it and a smaller object inside the disk. Is this the true scenario? No one knows! Image via Leigh Smith/ YouTube.
Remember Tabby’s Star? Citizen scientists with the Planet Hunters project spotted it in 2015. Tabby’s Star was dimming rapidly, in weird and unexpected ways. The theories for its unusual behavior ranged from alien megastructures to clouds of dust. That mystery hasn’t been fully solved, but dust does now seem to be the culprit. In June 2021, astronomers released a new study about a new and even bigger mystery (literally). It’s a huge and opaque “something” that dimmed the brightness of a giant star by up to 97% over a period of about 200 days. Even the largest dips in the brightness of Tabby’s Star reached only about 22%. Is it dust again? What else could it be?
The star, VVV-WIT-08, is an evolved giant star, some 100 times larger than our sun. It’s older than our sun, too, about 8 billion years old in contrast to our sun’s 4 1/2 billion years. It lies approximately 25,000 light-years away in the direction of the constellation Sagittarius, toward the dense center of our Milky Way galaxy.
Its minimum brightness happened in April 2012. Scientists still don’t have an explanation.
But they’re still thinking about it. The journal Monthly Notices of the Royal Astronomical Society published a new study about this star on June 11, 2021. See the preprint here.
‘Something’ dims a giant star
It’s a fascinating mystery, since whatever blotted out the star doesn’t seem to have the properties of an ordinary dust cloud. As described in the new paper, whatever it was had a “hard edge” and was virtually completely opaque.
Astronomers first noticed the weird event in data from the VISTA Variables in the Via Lactea (VVV) survey. As lead author Leigh Smith of the University of Cambridge told The Guardian on June 11:
It appeared to come out of nowhere.
It’s unusual for a star to dim in brightness by this much and for this long.
Another team member, Sergey Koposov of the University of Edinburgh, commented in the scientists’ statement that:
Astronomer Leigh Smith at the University of Cambridge led the new research on the strange dimming of giant star VVV-WIT-08. Image via LinkedIn. Image sequence showing the star at normal brightness in 2010, then dimmed up to 97% in 2012, and then back to normal brightness in 2013. Image via ESO/ The Guardian.
It’s amazing that we just observed a dark, large and elongated object pass between us and the distant star, and we can only speculate what its origin is.
They saw it happen only once
Seeing such a dramatic dimming, with such an unknown and mysterious cause, is both interesting and frustrating for astronomers. They see that the drop in brightness is impressive: 97%! And they’d like to be able to puzzle out how such a dramatic brightness drop could take place. These astronomers used data from the European Space Agency’s Gaia spacecraft and the ground-based survey called OGLE to try to solve the mystery.
But there are many, many possible scenarios suggested by the data. With Tabby’s Star, for example, the dimming differed with different wavelengths, which is compatible with clouds of dust. But with VVV-WIT-08, the consistent dimming across all wavelengths shows that the object was much more opaque and “solid.”
Another oddity was that the observations from Gaia even suggested that the star was moving through space faster than previously thought, almost fast enough to escape the Milky Way.
That doesn’t make sense for a star located in the direction of the galactic bulge, in the middle of our galaxy.
Many theories, but no answers
Astronomers said that – to try to explain the mysterious event – they’ve considered and then rejected many possible theories. The object appeared to be larger than the star itself, but could it have been smaller than thought and closer to Earth? Might it have drifted in front of the star, from our viewpoint, by chance?
That kind of chance alignment, as seen from Earth, would be unlikely, the researchers said. If it happened for this star, shouldn’t it happen for others? That would mean there should be many dark, floating objects in the space of our galaxy. If so, why haven’t we seen them before this? As mentioned in the paper:
We find that an occultation [the hiding of one object by another] due to a chance alignment with the giant star requires an improbably large space density of dark foreground objects. Hence, the occulter [the object doing the hiding] is most likely gravitationally bound to the giant.
Could the dimming have instead originated from the star itself? Stars that dim and then get bright again are common. But giant stars like VVV-WIT-08 don’t tend to drop in brightness by 97%. As noted in the paper:
The behavior of the light curve of VVV-WIT-08 does not correspond to any known intrinsic stellar variability. It is almost certainly an occultation of the giant star.
Right now, the most plausible conventional explanation for VVV-WIT-08 would be a giant debris disk of dust and small rocks that orbits a still-unknown body that, in turn, orbits the star. Image via NASA/ JPL-Caltech/ Jacob White.
Could it be a debris disk?
The researchers say the object that caused the dimming is probably gravitionally bound to the star. It might be orbiting the star. But what is it?
The best conventional explanation would likely involve dust. It might be some a huge, dusty debris disk. Such disks are common. They sometimes surround stars, the way that rings surround a planet like Saturn. Maybe the dimming of VVV-WIT-08 was caused by a debris disk. Maybe the disk wasn’t orbiting VVV-WIT-08 itself, but another body that in turn is orbiting VVV-WIT-08. In other words, maybe VVV-WIT-08 has a companion star or planet with a large, dusty debris disk.
Dust is usually easily recognizable, however, as it filters light in a way that allows longer, redder wavelengths to pass through it. The data from VVV-WIT-08 didn’t look like that.
Debris disks also don’t have “hard edges” as such. With dust, you typically see a tapering off at the edges.
Another possibility though is a giant ring system around the orbiting body. Astronomer Jason Wright at Pennsylvania State University noted in National Geographic that Saturn’s rings have well-defined edges. That definition is thanks in part to the gravitational influence of small moonlets that orbit in the gaps between the rings.
This object would likely have to be one giant ring though, since no gaps were observed. The star was seemingly blotted out by one large continuous mass.
If it’s a debris disk, then what’s it orbiting?
Maybe the object hiding the star is a debris disk. But, if so, we still don’t know what the debris disk is orbiting. Main sequence stars, like our sun, and white dwarf stars have been know to have debris disks. But those kinds of disks don’t fit with what’s been observed at VVV-WIT-08.
Or, here’s another dust scenario that doesn’t fully fit, but is still on the table. The researchers considered an orbiting body that is stripping dust off of VVV-WIT-08.
The mysterious dimming of VVV-WIT-08 is reminiscent of Tabby’s Star in some ways (artist’s concept). But the dust around Tabby’s Star is not nearly as opaque as whatever blotted out this other giant star. Image via NASA/ JPL-Caltech/ Sky & Telescope.
Could it be a black hole?
Another, more exotic possibility is that the orbiting body is a black hole with a dense debris ring around it. Intriguing, although it’s something that hasn’t been seen before. From the paper:
We considered a number of possible astrophysical objects as candidates for the occulter. Debris disks around main sequence stars are too optically thin. While white dwarf debris disks are optically thick, they are too small. Accretion disks around black holes and neutron stars usually emit X-rays, but a black hole fallback disk of the type described by Perna et al. (2014) might plausibly explain the occultation.
‘Blinking giant’ stars
Astronomers also think it’s possible that VVV-WIT-08 might belong to a new class of “blinking giant” stars.
These stars have been observed to be eclipsed by huge disks of dust, although not exactly the same as VVV-WIT-08. The star Epsilon Aurigae is eclipsed every 27 years, but only by about 50%. Another giant star, TYC 2505-672-1, is eclipsed every 69 years. The orbital period for whatever blotted out VVV-WIT-08 is still unknown, but must be at minimum 9 years, since another dimming episode hasn’t been seen yet since 2012.
Philip Lucas from the University of Hertfordshire said in the statement from University of Cambridge:
Occasionally we find variable stars that don’t fit into any established category, which we call ‘what-is-this?’, or ‘WIT’ objects. We really don’t know how these blinking giants came to be. It’s exciting to see such discoveries from VVV after so many years planning and gathering the data.
Giant stars often release material into space, but it’s the specific nature of the material near VVV-WIT-08 that has astronomers so puzzled. As Levesque told National Geographic:
It’s nicely not too bizarre it’s the sort of thing that you would expect. But dust does not look this neat, and it would certainly imply something very unusual about how that dust is distributed.
If these indeed are a new type of “blinking giant” stars, then astronomers expect to find more of them, according to Smith:
There are certainly more to be found, but the challenge now is in figuring out what the hidden companions are, and how they came to be surrounded by discs, despite orbiting so far from the giant star. In doing so, we might learn something new about how these kinds of systems evolve.
Last, but not least, could it be artificial?
Of course, speculation also includes the possibility of this being an artificial object or objects orbiting the star. Dyson spheres come to mind. Proponents of solar power know that only a tiny fraction of the sun’s total energy strikes the Earth. What if we, as a civilization, could collect all of the sun’s energy? That would require solar power collectors in space, likely in a shell around our star, or around our planet.
If we built such a thing, we would be using some form of Dyson sphere, sometimes referred to as a Dyson shell or megastructure.
Proving the existence of an artificial structure in the vicinity of a distant star would be monumental. It would mean there are advanced civilizations elsewhere in our galaxy.
But, as always, astronomers will tell you we shouldn’t jump to this conclusion, every time we find something strange.
And so the search for answers continues!
The most exciting possibility for VVV-WIT-08 would be something artificial orbiting the star, something like a Dyson sphere (artist’s concept). But whatever caused the dimming seems to be orbiting the star, not surrounding it as a Dyson sphere would. Image via CapnHack/ energyphysics.wikispaces.com.
Bottom line: Something huge and completely opaque blotted out the giant star VVV-WIT-08 for 200 days, with minimum light occurring in April 2012. Scientists still don’t know for sure what it was. A new study raises more questions than answers.
On Thursday night, July 3, the bright star Antares will appear to the lower right of the waxing nearly-full Moon.
Saturday morning, July 4, at 7:35 AM EDT, the Earth will be at aphelion, its farthest from the Sun in its year-long orbit. The Earth will be 3.4% farther from the Sun than it was at perihelion in early January. Since light intensity drops off as the square of the distance from the light source, the sunlight reaching the Earth at aphelion will be about 6.5% less bright than sunlight reaching the Earth at perihelion.