Transits of Earth from Other Planets

Credit: NASA

News media over the past few days have been tiled over with stories prompted by the transit of Venus later today, when the planet crosses in front of the sun’s disk. The event by all means deserves the attention, given its rarity, its historical and ongoing scientific importance, and the colorful adventures that have sometimes followed quests to watch it. I’ve even contributed to the glut myself with my SmartPlanet column from last week about how the transit of Venus relates to the search for worlds around other stars.

But let’s turn away from Venus for a moment and consider a parallel event that has never yet been seen; one that is paradoxically both more remote and closer to home: the transit of Earth as viewed from other planets.

All the planets of our solar system except Mercury and Venus have opportunities to see transits of Earth, at least in principle. (Mercury and Venus, of course, do not because they lie between the sun and Earth’s orbit, which is why we can see their transits.) And if any of the more than 2,300 planet candidates identified by the Kepler space observatory have civilizations that lofted their own versions of the Kepler, they are in a position to see transits of Earth, too: by definition, their orbits must sometimes align with the plane of Earth’s. [Note that Robert in comments says this last point is incorrect.]

Unfortunately, the quality of the spectacle drops off sharply with distance from the sun. For those planets in other solar systems, a transit of Earth would be nothing more than a brief, tiny dip in the brightness of the starlight from our sun, and it wouldn’t be too much better from much of our outer solar system.

Still, for observers on a sweet spot like Mars, the sight could be amazing. Fundamentally, it would look like a scaled-down version of a transit of Venus, but what could make it extra interesting is the presence of our inordinately big Moon. The fifth largest satellite in the solar system, the Moon’s diameter (2,160 miles) is more than a quarter the size of Earth’s. (Only Pluto’s moon Charon is more outsized relative to its primary: its 750-mile diameter is more than half of Pluto’s.) Depending on the relative positions of Earth, the Moon, and the viewing planet, during the transit the Moon could be a second black dot on the sun either preceding or lagging the main one by up to 30 Earth diameters. Or it could overlap Earth’s disk and make something that looked like a black snowman. Or, in a more glancing transit, it could miss the sun altogether. In short, transits of Earth are much more varied in appearance than those of Venus or Mercury can be.

So when will transits of Earth happen where?

Transits of Earth from Mars

It will not have escaped your attention that humans are not currently on any other planets, so if a transit of Earth were about to occur anywhere soon, we could not see it. The one noteworthy exception, though, is Mars

For an astronomical transit to occur, the sun, the transiting planet, and the viewing planet all have to precisely align. Because of differences in the speeds and planes of their orbits, such alignments are rare but are mathematically calculable from this formula:

1/ (1/P – 1/Q)

where P is Earth’s sidereal orbital period (365.26 days, the amount of time it takes for Earth to complete one orbit with respect to the fixed stars) and Q is the orbital period of the viewing planet.

Credit: NASA

Just as transits of Venus that we see on Earth follow a complicated 243-year cycle (with appearances separated by eight years, 121.5 years, another eight years, and 105.5 years), transits of Earth from Mars follow a 284-year cycle. The spacings are 100.5 years, 79 years, 25.5 years, and another 79 years. For better or worse, we are currently amidst that 100.5-year interval: the last one was May 11, 1984 and the next one won’t happen until November 10, 2084. If any of the rough plans for sending humans to Mars come to fruition, then by 2084 someone ought to be able to see it.

Here’s a YouTube video that offers some sense of what that person might see:

Transit of Earth from Mars

Transit of Deimos, as captured by the Mars rover Opportunity. (Credit: NASA)

In a sense, though, it’s too bad that a transit of Earth isn’t happening much sooner because in theory we could watch it with one of the robotic probes we have been sending. The Spirit and Opportunity rovers were able to observe the transit of the Martian moon Deimos across the sun in 2004. If their cameras had supported higher resolutions, they could also have watched a transit of Mercury in 2005. The rover would certainly be able to see the transit of Earth because depending on Mars’s distance from the planet, Earth would have an angular diameter between about 50″ of arc 17′ (a bit more than half as big as a full moon looks to us) and 3.5″ (half again as big as Deimos looks from the surface). [Corrected, with thanks to David and George Flanagin in comments.] Alas, that sighting is not to be.

But here are a few more upcoming transits of Earth, in case you expect that your descendants will be Mars colonists (or you have a very robust health plan):

  • November 15, 2163
  • May 10, 2189
  • May 13, 2268
  • November 13, 2368

You can see a table with more dates all the way out to 3015 in this 1983 article that appeared in the Journal of the British Astronomical Association.

By the way, if you want to watch something really amazing and have half a million years to kill, you can look forward to seeing simultaneous transits of Venus and of Earth from Mars in the year 571,471.

Transits of Earth from Jupiter

The sight of a transit of Earth gets less compelling as one moves farther out into the solar system, but there is one small compensation: the transits often get more frequent and closer together. I haven’t seen an explanation for this pattern yet but I’m guessing that because the outer planets orbit more slowly, their positions shift relatively less as Earth laps them, which creates more opportunities for alignments to be recreated. (If someone knows otherwise, please explain in the comments.) It might also be that simply because of the huge size of the planets in the outer solar system, there’s a much wider range of potential viewing angles, and near misses at the equator could become grazing transits at the poles.

Over the next century, transits of Earth from Jupiter will occur on:

  • January 5, 2014
  • January 10, 2026
  • June 24, 2055
  • June 29, 2067
  • December 26, 2072
  • July 4, 2079
  • December 31, 2084
  • July 9, 2091
  • January 4, 2097
  • January 10, 2109

Also, on Dec. 21, 2060, Earth will just miss crossing the sun but the Moon will graze the edge of it.

But if you’re thinking of booking ahead with Virgin Galactic, remember there’s a snag. Jupiter is a gas giant that probably lacks a distinct surface, so there’s no place to stand to view the transit. Moreover, at any significant depth within the atmosphere, murkiness (and the sun’s already dimmer light) will make viewing difficult if not impossible.

The best hope for viewing a transit of Earth from Jupiter would therefore really be from Ganymede, Callisto, or one of Jupiter’s other moons. Of course, all of those moons have their own orbital motions, so sometimes even when Jupiter has a view of a transit of Earth, a particular moon’s line of sight will be obstructed by the planet.

Oddly enough, though, we may have another way to view the 2014 transit even though we don’t have any spacecraft there. As astronomer Jay M. Pasachoff of Williams College noted last month in an article for Nature, it should be possible to capture the reflection of the transit of Earth off of Jupiter’s clouds by training the Hubble Space Telescope on them. Because Hubble viewing time is in high demand among astronomers, it’s not certain that this will be done, but it’s would be a fantastic opportunity to capture a sight that will otherwise be elusive for many more decades.

Transits of Earth from Saturn

Let’s face it: if you’re in the vicinity of Saturn, nothing that Earth can do can possibly compare to the incredible sight of the planet’s rings. Saturn is also another gas giant, which means that Titan or one of the other moons would probably offer the best viewing opportunities, subject to orbital motions and local weather conditions. (Imagine traveling all the way to Titan to watch a transit of Earth and then getting rained out by a passing methane storm.)

We may have had an opportunity to watch a transit of Earth from near Saturn a few years ago but passed it by. The Cassini spacecraft was there in 2005 for the most recent one—but as luck would have it, the transit seems to have coincided with the date of the deployment of the Huygens probe down to the surface of Titan. Redirecting Cassini’s cameras toward the sun was therefore a much lower priority, particularly given that Earth at that distance would have been, at best, at the edge of the probe’s visual resolution.

Nevertheless, future transits of Earth from Saturn through the end of the century will happen on:

  • July 20, 2020
  • July 16, 2049
  • January 16, 2064
  • July 11, 2078
  • January 9, 2093

Transits of Earth from Uranus and Neptune

Viewing transits of Earth from the even more distant gas giants becomes an increasingly fruitless exercise. The sun is only a small disk in the sky and Earth’s passage across it would only be visible with a fairly powerful telescope. Once again, the planets’ moons would be the best bases for observations, assuming they were in the right positions.

Uranus at least offers the most theoretical opportunities for watching one this century because every 40 years or so has a batch of eight or nine transits in fairly rapid succession, while the planet is moving through the plane of Earth’s orbital ecliptic:

  • November 17, 2024
  • November 21, 2025
  • November 26, 2026
  • November 30, 2027
  • December 3, 2028
  • December 8, 2029
  • December 12, 2030
  • December 17, 2031
  • December 21, 2032
  • May 19, 2065
  • May 24, 2066
  • May 29, 2067
  • June 2, 2068
  • June 7, 2069
  • June 12, 2070
  • June 17, 2071
  • June 20, 2072

Transits seen from Neptune are similarly clustered except that the sets are about 80 years apart. In this century, remaining opportunities include:

  • January 23, 2081
  • January 25, 2082
  • January 28, 2083
  • January 30, 2084
  • February 1, 2085
  • February 3, 2086
  • February 6, 2087
  • February 8, 2088

What about Pluto?

The experience of looking for a transit of Earth from Pluto would be even feebler than those from Neptune. Worse, Pluto’s opportunities to experience a transit are much more limited because its orbit is highly inclined to the rest of the ecliptic—that is, it orbits in a plane that is angled at about 20° to the one that approximately holds the rest of the planets.

I haven’t found a specific listing for a transit as seen from Pluto, but Pluto should be crossing the ecliptic again in approximately 2018. Figure that there could be a chance for one around then before it pops back out the other side of the ecliptic, not to return to it for another 124 years or so. (Again, if anyone can offer more specific information, it would be warmly welcomed.)

(In researching this post, I’d like to acknowledge the assistance of Wikipedia and its surprisingly detailed information on this topic.)


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11 Responses to Transits of Earth from Other Planets

  1. Question says:

    Is the part about the Earth, when viewed from Mars, looking up to the half the size of the full moon accurate? It seems unlikely and contrary to what I’ve heard in the past.

  2. Robert says:

    The Kepler planets can never see Earth transit. To see Earth transit the Sun, an exoplanet must be less than a few degrees from the ecliptic, the projection of our orbital plane on the sky (this is the same path the Sun follows throughout the year). All of the Kepler targets are well outside this zone. We see them because we happen to be with a few degrees of what the inhabitants of a Kepler planet would define as the ecliptic in their sky.

  3. David says:

    Question: It’s not accurate. The maximum apparent diameter of Mars as viewed from Earth is 25.1 seconds of arc, but the maximum apparent diameter of the Moon is 1896 seconds. Source: NASA Planetary Fact Sheet”

  4. David says:

    Sorry, I got it backwards. We want the apparent diameter of Earth as viewed from Mars, not Mars from Earth. That should be more like 50 seconds of arc.

  5. Richb says:

    Every few years a crackpot predicts doom (earthquakes, etc) because all the planets are in a line. Seems to that truly could only be considered to occur when the other seven planets can be viewed as in transit from Neptune (ignoring Pluto and other sub planetary objects). Guessing from the rarity of even 2 planets being in transit from a third (Mercury and Venus seem to occur once every couple 100,000 years or so) that such an event would be once every many billion years. Unlikely to occur before the sun swallows the 2-4 inner planets. The other 3 rocky planets being visible in transit from Mars may be a rational event- maybe separated by 10-100 million years?????

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