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Syzygy - When Heavenly Bodies Align
If you were reading about what to look for in the sky, you might wonder if it's as negative as it sounds for Saturn to be at opposition. Or if a superior conjunction is better than an inferior conjunction. And although everybody knows about eclipses, what are transits and occultations?
Here's an explanation of these terms.
It's actually all to do with syzygy [SIZ-i-jee]. Syzygy is a funny-looking word, but in astronomy, syzygy is simply three (or more) heavenly bodies in a line. This happens twice a month with the Sun, Earth and Moon, which are lined up at new Moon and at full Moon.
Inferior and superior planets
We can describe another planet in terms of its position relative to Earth. It's not a judgment to describe a planet as inferior or superior, but simply refers to its orbit compared to Earth's. The inferior planets, Venus and Mercury, are closer to the Sun than we are. The superior planets, Mars and the giant planets, are farther away.
If Mercury or Venus is lined up with the Earth and Sun, the syzygy is a conjunction. If Mercury or Venus are on the far side of the Sun from us, they are at superior conjunction. But if they are between us and the Sun, its called inferior conjunction. Conjunctions are times when we can't see the inferior planets, because they're either behind the Sun or lost in its glare.
The rest of the planets are superior planets. They can't get between us and the Sun, so are never at inferior conjunction. But there is a conjunction if one of them is aligned with us and is on the other side of the Sun.
When a superior planet is aligned so that it's in the opposite side of the sky to the Sun, it's at opposition. The planet is then at its closest to Earth, so this is a good time for observing, as are the weeks around the opposition. Not only is the planet close, but it's rising around sunset and visible through the night.
In an eclipse one object passes in front of another and substantially hides it from us. For example, in a solar eclipse, the Moon blocks all or part of the Sun from our sight.
A lunar eclipse is slightly different, because we can't see our own planet block the Moon. What we see is Earth's shadow darkening the Moon. If we were on the Moon during a lunar eclipse, we'd see a solar eclipse with Earth hiding the Sun. On Earth during a total solar eclipse, the Moon blocks enough of the Sun's light that we can see the solar corona. However the Earth is much bigger than the Moon, so from the Moon we wouldn't see the corona.
We can think of a transit as a mini-eclipse, because one object passes in front of another. However the foreground object is (or seems to be) very small compared to the background object, so it doesn't do much eclipsing. Perhaps you saw a transit of Venus in 2004 or 2012 Venus was only a dot on the face of the Sun.
A certain number of eclipses usually partial ones are seen from space. Transits are rare, but astronaut Don Petit photographed the 2012 transit of Venus from the International Space Station. However on June 3, 2014 NASA's Curiosity rover on Mars observed a transit of Mercury. Since the transit wasn't visible from Earth, this was a unique observation.
Transits of Venus were historically important in determining the distance of the Earth from the Sun. Even today, they're not mere curiosities. The transit of a star by a planet causes a tiny dip in its light. Since 2009, the Kepler space telescope has been recording the light curves of tens of thousands of stars. After searching the data for light dips characteristic of a transiting planet, astronomers follow up planet candidates. In January 2015 the 1000th exoplanet was confirmed from Kepler data.
Unlike a transit, where a small object passes in front of a much larger one, the foreground object may seem to be much bigger than the background one. It won't just cover a little bit of it, but will hide all or a substantial part of it. This kind of eclipse is called an occultation.
Occultations can be useful to astronomers. For example, it's generally extremely difficult to measure the diameter of a small body in the outer Solar System. Yet when dwarf planet Eris occulted a star, the timings allowed astronomers to calculate an upper limit for its diameter. And even before Voyager 2 visited the outer Solar System, the rings of Uranus and hints of rings around Neptune were discovered through stellar occultations by these planets.
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