# Can the Earth be seen to transit the sun from the outer planets?

Transit of Mercury as seen from Mars The Curiosity rover on the planet Mars observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth, on the 3rd June 2014.

This question may be answered by astronomy software, that's the reason I think there may be an answer. I have some similar programs, but none has this option.

Transit of Mercury from Earth.

I ask this question, can the Earth (actually the Earth Moon system), be viewed in transit across the sun, from pure curiosity, as I can't imagine it has much practical value, although I don't know enough about the subject to judge that. Possibly it gives an indication of the "flatness" of the orbital planes of the outer planets, although I am sure that this has long since been established

It may only be answerable by computer generated data, although I do not want to underestimate the astronomical achievements of generations of astronomers, who may well have worked out the dates of Earth transits a long time ago.

• Sep 23, 2015 at 11:41
• As a sidenote to "to me this image this really brings home the relative size scales involved between the sun and it's planets", I would say that no it doesn't. Actually Mercury is closer to the camera than the Sun is and so it appears bigger with respect to the Sun than it really is. Sep 23, 2015 at 12:29
• @no_choice99 Mercury is at 0.387 a.u. from the sun, so it will be magnified by about 1.6x compared to the Sun. That means that Mercury is even smaller than the picture suggests - either way it gives a sense of the massive size of the Sun. Sep 23, 2015 at 13:10

Actually, any planet $X$ can be see to transit the sun from any other planet $Y$ with a larger orbit. That being said, the frequency for which the a particular transit happens tends to decrease as the planets you choose are further away from the Sun.

For example, here on Earth, the angular diameter of the Sun is about 32 arcminutes, or just slightly more than half a degree (although the exact angular diameter fluctuates a little depending on the Earth's position in its orbit). However, on Mars, the angular diameter of the Sun is only about 5/8 of that, or 0.35 degrees. As a result, the disc of the sun is a smaller "target" for a planet to transit across, and there's a greater chance that the planet will pass off to one side of the Sun's disc.

Note: I swear I have seen a table somewhere before showing the frequency of planetary transits from other planets. However, I can't find it at the moment, so I'll post what I have above as an answer, and try to fill in with a table once I can locate it.

Edit: This link shows that transits of Uranus, as seen from Neptune are the rarest of all planetary transits, for the reasons I mentioned above. I still can't find a single table, but if you look at the bottom of that webpage, there are links to every other possible transit combination for you to peruse.

• You can find predictions on Wikipedia. Having said that, I am a little in doubt about the generality of your first sentence. It seems to me that if two planets are in resonance and they are not orbiting in the same orbital plane, then the transits may be suppressed. If by "sun" you mean exclusively "sol" then this wouldn't be the case and you are right. Sep 23, 2015 at 12:10
• Yes, I am specifically describing transits of planets in our solar system, because we have such complete orbital data on each planet. But Jupiter's Galilean moons is a good example of a system where some of the moons are in resonance and would never appear to transit one another.
– Sean
Sep 23, 2015 at 12:15
• The generalization was all in my mind. Apologies! Sep 23, 2015 at 12:16