# Astronomy: Opposition question

Staff Emeritus
Gold Member
I could sure use some help with this question on my Astronomy homework:

When the moon is at opposition from the Sun, what time of the day does it rise in our sky?

To try to figure it out, I drew a picture with the objects in this order

Moon ---- Earth ---- Sun

I thought that maybe you couldn't see the moon because the Earth's
shadow would block all the light that it would reflect from the Sun.
But maybe I have the wrong understanding of "opposition" altogether.

I am an absolute beginner in this subject and any help is greatly appreciated!

Thank you!

## Answers and Replies

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Your picture is correct when looking from the ecliptical pole. But the moon's orbit is not within the ecliptic. It's at an angle with the ecliptic. And that's the reason why we normally see a full moon. And not an eclipse of the moon. The full moon is not precisely 'full'. And it does not precisely rise when the sun goes down. But very close so. Since the angle between the moon's and earth's orbit is small.

Staff Emeritus
Gold Member
Thanks for your response, arcnets
If the moon is at an angle...

then would I see the moon rise at about 6 PM when it's in opposition to the Sun?

. . . . . (moon)
................. 6 PM
.................. ^
.................. |
Midnight < - (me) - >Noon. . . . . . . . . . (sun)
.................. |
.................. v
.................. 6 AM

Or were you trying to tell me that the moon just CAN'T ever be in opposition the Sun because it's not within the ecliptic plane?
Sorry - slow learner here! Thanks!

HallsofIvy
Homework Helper
Your answer "6:00" is exactly right. Arcnet's remark about the "ecliptic" was only in regard to your concern about not being able to see the moon. Actually, sometimes the moon DOES cross the ecliptic when it is in opposition to the moon- the result is a "lunar eclipse". (Hmmm, "eclipse", "ecliptic" think there's some connection?)

When the moon is in opposition to the sun but not at the ecliptic, the sunlight "misses" the earth on its way to the moon but the entire face of the moon is open to it. Far from being unable to see it, that's the full moon!

Staff Emeritus
Gold Member
Cool! I think I am actually starting to "get it" now!
Thank you both so much!

Here's a another picture:

......................^...................
......................|...................
....................south.................
......................|...................
......................|...................
.....................YOU..................
<-----east----------[zz)]-----west------->
full moon...........earth.................sun
(rising)..............|...................(setting)
......................|...................
....................north.................
......................|...................
......................v...................

Plus, the moon is in general a little above or below the 'paper'.

Last edited:
Staff Emeritus
Gold Member
Thank you, arcnets. A picture's worth 10^3 words. Wish I could borrow your brain for my midterm tomorrow!