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# Would the inner planets of our Solar System line up?

2017-03-20 23:38:42

The protagonist in my SF story is on an asteroid gazing towards the Sun and inner planets. For purposes of the narrative he needs to locate the position of the earth at different times in its orbit.

He is standing on the asteroid, about 2.5 Astronomical Units out, in the asteroid belt, viewing towards the Sun

Since he’s basically outside looking in with regard to the Earth, wouldn’t all the inner planets line up in a straight line passing through the sun?

Wouldn’t the Earth (and Mercury, Venus, and Mars) move back and forth along that imaginary line (red arrows), their location along that line depending on what point they are in the orbit? For instance, Venus could appear to be inside Mercury, as in my drawing, correct?

Edit: the asteroid has little or no inclination.

What you described would more or less occur, provided the light from the sun can be blocked out, the planets would appear to line up. Over the course of the year(s) they would appear to move back

• What you described would more or less occur, provided the light from the sun can be blocked out, the planets would appear to line up. Over the course of the year(s) they would appear to move back and fourth. However, the planets would be much less visible, if at all, at their closest approaches to the observer because the sunlight would not be reflecting off of them.

2017-03-21 00:02:39
• Yes, that's approximately how it would look, with some caveats:

The asteroid is orbiting the sun, too, so your protagonist isn't a completely stationary observer (although it will orbit slower, since it is further out).

The sun is bright, so you may need to have something to block it to observe carefully.

Any planets closer to the sun than you will have phases, like we observe on Venus. The observer would be able to determine, then, whether the planet is in front or the back part of their orbit around the sun.

All the planets are very slightly tipped, so they likely won't actually transit (pass directly across) the sun on every orbit. Compare the frequency of transits of Venus and transits of Mercury.

Unless other asteriods come close, Earth's moon is probably the only other object large enough to distinguish, at least toward the sun.

2017-03-21 00:03:32