... and as Venus sets in the west ...

These nights, in the northern hemisphere, Venus is impossible to miss in the western sky not long after sunset.

I’ve been watching it these past few evenings and noticed it seems to be following the same path/arc the sun follows as it sets each day. I guess that makes sense because the sun and all the planets are more or less on the same plane.

Do I have that right? Do all the planets rise in the east and set in the west?

Yes. Every celestial object in the sky rises in the east and sets in the west as the earth rotates on its axis. The planets and the Moon have their own motions as well, causing, for example, Venus to sometimes appear as the evening star and sometimes as the morning star, but the daily rotation of the earth carries them all around us approximately once a day.

Well, west plus or minus 40 degrees or so…

The declinations of the planets (which, along with your latitude, determines where on the horizon they rise/set) are not all “more or less the same”, exactly: right now Venus is about 9 degrees north, while Saturn is 22 1/2 degrees South.

To be fair, the inclination of the planets’ orbits are not that far off from each other, but that is not what directly determines where they rise and set, since the Earth is tilted on its axis (think about where on the horizon the sun sets!)

On Earth, everything rises in the East and sets in the West. But on Mars, one of its moons (Deimos, I think) is so close to the planet that it actually orbits faster than the planet rotates, and so it rises in the West and sets in the East.

“The hurtling moons of Barsoom”, indeed!

Yup. What’s more, they all travel on roughly the same path through the sky as the sun and moon. This path is called the ecliptic.

If you were on the Moon or on another planet in the solar system, you would also see the planets confined to a narrow path through the sky. On some planets, you might see moons follow a different path.

We think that this is likely to be true of other solar systems as well. But bear in mind that it’s tricky to figure out the inclination of an extrasolar planet’s orbit, and thirty years ago we would have said that we wouldn’t expect to find giant planets near their stars. We have since found many such planets, so who knows what all is out there to find?

While we’re talking about moons: The discovery of 12 new ones for Jupiter has been announced. That gives a total of 79, by Jove.

Many of the new ones have orbits in the 1-2 (Earth) year range and are retrograde. So those set slowly in the east from a very hypothetical Jupiter point of view.

One has been touted to be on a course to collide with another moon. But that could take millions of years at the least. So no [del]Earth[/del] moon shattering kaboom for you.


Well, for a given value of “slowly”, since Jupiter’s rotation period is just under 10 hours.

Since the orbital period of the moon is much longer that the rotation period of Jupiter, surely it sets in the west (as seen from Jupiter) regardless of which way it orbits?

If Venus had moons, things might get interesting, since Venus rotates (very slowly) backwards relative to all the other planets in the Solar System.

As Jupiter is not a solid body, the question is where would you choose to decide that? The rotation period is defined based on the the Magnetosphere so I guess we could use that.

While the new ones are slower Jupiter has two known moons that actually do orbit faster than rotation Metis and Adrastea, with Mars being the only other planet with such a Moon.

Even Io which has a period of 52 hours visibly moves over a viewing session in my very small 6" CAT.

But I am curious what the Sidereal rotation period is of the Earths upper atmosphere, but I am failing on search terms as I can only find older documents that show that even the Earths upper atmosphere rotates faster than the surface which is interesting.


It can be argued that the Earth Moon system is better described as a binary system compared to the moons of Jupiter and Mars but It is interesting that it apparently appears to rotate faster (or they thought in 1966) Which seems to be the opposite of what I typically read and would assume, especially with the transfer of momentum due to tidal interaction with the moon.

For those who don’t want to read through the math, it appears that the atmosphere 200-300km above the surface rotates 1.3 times faster than the surface (according to that paper)

*s a consequence, as seen from the Martian equator, Phobos would appear to move far more rapidly than Deimos. In fact, Phobos would already be moving overhead just 2 hours and 48 minutes after rising. And after another 2 hours and 48 minutes, it would already setting. An astronaut on Mars could therefore witness Phobos rising twice during a single night.

And since Phobos’ west-to-east motion is much faster than Mars’ rotation period, the satellite would appear to rise in the west and set in the east.

Furthermore, about every 10 hours and 18 minutes, Phobos would appear to race closely past Deimos as the two moons trekked in opposite directions. Phobos, in fact, would probably even briefly eclipse Deimos as seen from some parts of Mars on each pass.

Try picturing this: During the 66 hours that Deimos moves ponderously in the sky toward the west, Phobos whizzes rapidly in the opposite direction more than six times!*

Since they have yet to define “moon” Jupiter likely has tens of thousands of “moons” some about the size of a baseball.

Saturn would have uncounted millions.

I would say almost, but the center is still within the earth.

In an attempt to avoid a derailment, the moon orbits the sun in what is best approximated by a circle, where most moons will have a concave section to their orbit in relation to the sun. There is a whole thread that could happen based on terms, but I am just stating that and that it makes it hard to make similar assumptions when talking about other planets satellites.



Because the sun’s gravitational effect on the Moon is about twice that of the Earths it can be problematic when trying to visualize these other systems, which will manifest in subtly different ways.

There is a more common definition of binary planets which relates to the barycenter being external to the bodies, and I am not suggesting that is an invalid definition.

The fact that the Moons orbit of the sun is convex is interesting.

Here we go again … the riots are just now started to settle down about Pluto and here we’re starting talking about demoting moons … hydrostatic equilibrium … that’ll eliminate a mess of moons …


Venus rotates around it’s axis backwards … but objects will still rise in the east and set in the west from a Vensian’s perspective, just by how we define north … but from every other planet’s perspective, the objects would seem to rise in the west and set in the east there …

Unanus’ axis is almost aligned with the ecliptic … crazy things happen there …

“Call me but lunar, and I’ll be new baptized”

As no moons are named after beloved Disney characters I doubt the response will be as robust.

I can’t believe no-one has mentioned the fact that Mars is in opposition yet. About 1:00 every morning this month, Mars is shining brighter than it has for 15 years, low down in the south. If you have clear skies and insomnia, it is well worth catching.