Saturday, March 7, 2020

The Lunar Eclipse... from Mercury

lunar eclipse MESSENGER
Mercury (green) was nearly between the Sun (yellow) and Earth (blue) at the time of the eclipse.
The Lunar Eclipse... from Mercury.

When the moon winds out and then winks back in - that is the lunar eclipse.
There is so much hardware floating around out in space these days that it is hard to keep track of it all. There are landers all over the place, on moons, on planets, and just floating around looking for somewhere to go.

There are probes to asteroids and craft leaving the Solar System. There are NASA landers on Mars and the International Space Station. There is a Chinese lander on the Moon and a probe circling Mercury, and...

Wait a minute, did I just say Mercury?

Today's animation comes from a probe that is rarely mentioned by anyone because Mercury is not that sexy. There are no famous books about little Mercury men who come to earth and start blasting away with ray guns.

MESSENGER (an acronym of MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched in 2004 to study Mercury. It took its sweet time getting there, but get there it did in 2011. MESSENGER has been in orbit around Mercury ever since, doing basic science research and taking lots of boring photographs. I say "boring" because there are no rocks with faces that can be seen from orbit or canals or anything like that. What we can see of Mercury is just a big rock, basically, and rocks are just not that intriguing, especially when they don't have a giant spot like Jupiter or anything like that.

MESSENGER's studies of the Mercury poles and core and all that is not our subject anyway. Instead, it is probably the most interesting thing that the probe has done in its ten years of existence, and it was purely by chance.

Normally MESSENGER looks straight down on the tiny world, mapping the terrain that slides underneath it with various cloud-piercing instruments (actually, it is just an exosphere, but that's fancy science jargon for kinda-clouds). But mission technicians saw an opportunity for something interesting, so they pointed the camera toward Earth and took 31 images, each two minutes apart, to capture the earth and the moon.

That still doesn't sound that interesting, because we see the earth and the moon ever day (we're standing on the former and looking up at the latter). In these 31 photographs, though, the moon disappeared.

Yes, that's right, the moon disappears. That is because the shots were taken during a lunar eclipse, meaning the earth's shadow covered the moon. This isn't all that rare, but it still is neat to see it from another world, namely Mercury (orbit). It is very cool for some of us to see actual images - not "recreations" or "imaginations" or anything like that, but the real deal that actually happened - from so far away. This is what an intergalactic traveler would see as he moseyed randomly about the system, looking for xenon gas to take back home for his cenobite factories. In other words, this has nothing to do with us whatsoever, but is simply a random cosmic event that happens routinely but is certainly noticeable by anyone who sees it.

You can see the Earth on the left, all of five pixels wide in the original images (the entire video has been expanded by a factor of two), and the Moon on the right, just barely bigger than a single pixel. The motion of the Moon is too small to detect, but as it passes into Earth’s shadow it dims considerably, disappearing.

Mercury was in a particularly good spot to see this, being at a narrow-angle from an imaginary line drawn through the sun and the earth. Thus, you see the moon swinging behind the earth and getting blotted out, but not from such an angle that you lose sight of the moon as it does go behind the earth. It is a good cinematic angle.

The one big omission is that they do not show the moon reappears. That would have made the animation 10 times better, but for some reason, that part is not included. So, we just see the moon disappearing and have to assume that it reappears a few minutes later (it did).

Even then, the brightness of the Moon has been multiplied by 25 to make the change more obvious. On Oct. 8, during the eclipse, Mercury was nearly between the Earth and Sun, so to MESSENGER, the Earth and Moon were close to full. But the Earth is bigger and more reflective than the Moon and would look 50 or so times brighter, so it made sense to enhance the Moon’s brightness.

MESSENGER was 107 million kilometers (66 million miles) from Earth when it took these images.


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