Earth setting on the Moon?
Published by admin March 3rd, 2008 in MoonWhat’s wrong with this picture?
It is featured here with the caption “Here is a series of images showing the Earth setting.”
As far as I know the Earth does not set or rise on the Moon.1 This is ensured by the fact that we see only one side of the moon. As seen from the Moon the Earth would be stationary on the sky with a slight wobble due to libration.
The photo was taken by Japan Space Agency’s Kaguya spacecraft. Video here.
My guess is that as the spacecraft orbited the Moon the Earth appeared to set because of the motion of the satellite. If this is so then it is misleading and wrong to claim that this picture shows the Earth setting on the Moon.
Can anyone correct if I am wrong?
- March 14, 2008 — See this thread on Astronomy magazine forum for definitive answers and correction.
Either in the pictures or in video, one can see that the position of the spacecraft is changing. But I don’t know if this difference in position is enough for say the “Earth set” is due to what you said.
It’s all a matter of the definitions of the words. Clearly the spacecraft is moving.
Alireza,
Do you agree that the Earth does not set or rise on the Moon? If so, this photo is misleading. It gives the impression that Earth sets on the Moon for a stationary observer.
Carl,
I think “set” has one meaning. It is the motion of the Moon, or Sun, or any other celestial body, due to the motion of the Earth. In the case of the Moon, the Earth does not set on the Moon. In this picture it is suggested that the Earth sets on the Moon the way the Moon sets on the Earth. If you know anyone who is not that familiar with astronomy may be you can ask them to look at this picture. I bet that they will say the earth sets on the Moon not because the spacecraft moves but because the Earth sets on the Moon the way the Moon sets on the Earth.
I agree with you Pioneer1.
Have a look here:
http://en.wikipedia.org/wiki/Libration
and you’ll see that it -IS- possible for someone on the surface of the Moon to see the Earth set. You’d have to be on the edge, though, to see the event. You can also see the variation in size as well as orientation that the Moon goes through in a single month. All the non-astronomical public I’ve shown this animation to have been amazed by it.
Yes, you’re right, though, about the JAXA Kaguya images, they aren’t really of an Earthset event, it’s an occultation. No biggie, nothing to loose sleep over. It’s a minor linguistic point.
The images -DO- show how dark the Moon is compared to the Earth. It seems so white & bright when we see it at night, though, huh? But put the Earth & Moon in the same image and you see the difference.
Richard B. Drumm
President
Charlottesville Astronomical Society
You are right again, Pioneer. Nice posts too.
Thanks, Louise.
Richard,
Thanks for the comment and the clarification. I added a footnote to the post referring to your comment.
Maybe you can correct my numbers but still not convinced that the setting would look like in this photo even if you are viewing it from the “edge.”
As seen from the Moon the Earth subtends about 4 degrees, the libration if I remember correctly is about 5.5 degrees. So the Earth would barely move its own disk. The picture shows the earth moving at least 3 disks.
Libration animation is awesome but at present the page is blanked for some reason.
Yes, minor linguistic problem, nothing to lose sleep over. By the way, I loved the Venus occultation photos that you have on your website