Astronomy: Opposition question

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In summary, the conversation discusses the question of when the moon rises in our sky during opposition from the sun and the concept of "ecliptic." The correct answer is 6:00 PM and the moon's orbit is at an angle with the ecliptic, causing it to usually appear full and not in an eclipse. The conversation also includes diagrams and gratitude for help.
  • #1
Math Is Hard
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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!
 
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  • #2
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.
 
  • #3
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!
 
  • #4
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!
 
  • #5
Cool! I think I am actually starting to "get it" now!
Thank you both so much!
 
  • #6
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:
  • #7
Thank you, arcnets. A picture's worth 10^3 words. Wish I could borrow your brain for my midterm tomorrow!
 

1. What is an opposition in astronomy?

An opposition in astronomy is when a celestial object, such as a planet or asteroid, is on the opposite side of the Earth from the Sun. This means that the Earth, the Sun, and the object are all in a straight line, with the Earth in the middle.

2. How often do oppositions occur?

The frequency of oppositions depends on the orbital period of the celestial object. For example, oppositions of Mars occur approximately every 2 years, while oppositions of Jupiter occur every 13 months.

3. Why are oppositions important in astronomy?

Oppositions allow astronomers to observe celestial objects in their full illuminated phase, which can provide valuable information about their surface features and atmospheric conditions. It also allows for more accurate measurements of the object's distance from Earth.

4. Can any celestial object have an opposition?

No, only objects with orbits that are exterior to Earth's orbit can have an opposition. This includes planets, asteroids, and comets. Objects with orbits interior to Earth's, such as Mercury and Venus, can have an inferior conjunction with the Sun, but not an opposition.

5. How can I observe an opposition?

You can observe an opposition by looking towards the eastern horizon just after sunset or the western horizon before sunrise, depending on the direction of the object's opposition. It is also recommended to use a telescope or binoculars for a better view.

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