Planetplanet Eclipses

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For the sake of completeness, there are a couple of other phenomena we might tidy up in our survey of peculiar types of eclipse. The first is trivial. In the Space Age a host of artificial satellites has joined our natural satellite, the Moon, in orbit about the Earth. These are eclipsed frequently. The time to watch for satellites is soon before dawn or just after dusk (because during the deep night, satellites in low orbits are within the terrestrial shadow, in eclipse). Far enough up that the Sun is still catching them, satellites in low orbits such as the space shuttle, the space station, or the Hubble Space Telescope typically take 90 minutes to circuit the planet. Those are only a few hundred miles up, higher paths taking longer to complete an orbit. The time to move from horizon to horizon typically is only a few minutes, but often one will see a satellite abruptly disappear, as it enters the shadow zone.

Devotees of satellite spotting also enjoy solar eclipses. In that situation the name of the game is predicting when a particular satellite visible in daytime (usually with binoculars) is going to pass into the shadow of the Moon—and then watch it actually happen. Catching artificial satellites being eclipsed, though, is a specialized modern-day sport. Let us return to natural events.

We have considered the Moon and planets crossing the Sun or the stars, Jupiter eclipsing the Galilean satellites, and measuring the sizes of asteroids and comets. Is it possible, though, that one planet could eclipse another? Venus, say, could cross the face of Jupiter, and because the former appears smaller than the latter this could be classed as a transit. Such an event might be seen around dawn or dusk if it happened that Venus were near maximum elongation (the greatest angular distance it achieves from the Sun) and Jupiter, on the opposite side of the Sun to the Earth, happened to line up. Alternatively Mercury might pass behind Venus and be occulted. Such things must happen—but not very often.

There is a thin line of differentiation between an occultation and a transit. One might say that a Galilean satellite is occulted when it passes behind Jupiter, but is in transit when it moves across the Jovian disk as seen from the Earth, the somewhat different direction to the Sun causing its shadow to be located elsewhere on that disk (see Figure 13-5). Both events might be thought of as forms of eclipse, which is why they merit mention.

The planets all orbit the Sun in the same direction, with orbital planes inclined slightly to the ecliptic. This prohibits planetplanet eclipses from occurring every year, but makes their occurrence more frequent than if they sped around the Sun with random orientations. Just how often do such events occur? As a long-term average, there are 7 or 8 years between solar transits of Mercury, and solar transits ofVenus occur once every 60 years. The Sun covers a much larger target area than any of the planets, so one might anticipate that transits of one planet across the face of another would be rare birds indeed. This is indeed the case.

In 1591, while still a student at Tübingen in Germany, Johann Kepler ventured out into a cold January night with his teacher to

FIGURE 13-5. A transit of Io, one of the four Galilean moons of Jupiter, across the face of the planet results also in a form of eclipse, with its shadow being cast on the cloud tops below.

observe a predicted close conjunction between Mars and Jupiter. To their astonishment only one reddish spot could be seen in the sky, and they surmised that the two had aligned with each other. This would be the first planet-planet eclipse observed, except that precise backward computations show that Kepler's senses must have deceived him. What actually occurred is called an appulse,

Mars and Jupiter passing very close to each other, like a grazing occultation. Without a telescope, two decades before Galileo opened the heavens to closer inspection, the human eye was inadequate to differentiate the adjacent pair of tiny planetary disks.

In all recorded history there is only one definite observation of a planet-planet eclipse, and that was watched by but one man, from the Royal Greenwich Observatory in 1737. John Bevis was a physician from a country area a hundred miles west of London, who had done well for himself in the city, giving him the time and money to pursue his amateur scientific interests, including astronomy. Although not on the staff, he often observed the heavens from the great observatory, his expertise with the telescopes being well recognized.

One evening he was observing with a rather crude refractor (a lens telescope), with a focal length of 24 feet. In those days such simple telescopes tended to produce poor images with colored fringes around celestial objects. Through the long tube of this ungainly instrument Bevis saw a gibbous Mercury and narrow crescent Venus near each other in the sky, and rapidly closing. (Both planets display phases like the Moon; "gibbous" is the phase between when a half and a full disk is illuminated.) Clouds intervened, and it was eight minutes before Bevis could again espy the brilliant but slender Venus. The dimmer Mercury he no longer could detect. He surmised thatVenus had eclipsed Mercury, but he was prohibited from seeing the smaller body emerge from behind the larger by yet more clouds, which blanketed the sky until the planets set in the west.

We met Urbain Le Verrier in the previous chapter, as one of the predictors of the existence of Neptune. Having been appointed Director of the Paris Observatory, in the mid-1800s he had drawn up tables of planetary positions and wanted to test their accuracy. Le Verrier seized upon Bevis's report from more than a century before as a stringent test. Sure enough he found the alignment would have occurred as described, with the slight refinement that Mercury was not completely covered by Venus. That part of Mercury protruding, though, was the dark part shadowed from the Sun, so Bevis could not have seen it in the glare of Venus. Bevis's northerly location was also critical; if he had been on the equator, the two planets would have swept past each other in an appulse.

A handful of other opportunities to witness planet-planet eclipses have been missed by astronomers over the past five centuries. In 1570 and 1818 Venus skimmed over Jupiter, but neither event seems to have been noticed. In 1705 an observer in Japan could have witnesssed Mercury practically touch Jupiter, as seen in the night sky, but it seems that none did.

Looking into the future, in 2037 Mercury will pass very close by Saturn, but not quite transit its disk or rings. If you choose your location carefully (go north, young men and women), you may see Mercury blotting out Neptune in 2067, but you'll need a decent telescope. After that, there is a transit of Venus over Jupiter in 2123, and in 2223 Mars will do likewise. Planet-planet eclipses, then, do not occur often.

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