SkyEye

Not from the stars do I my judgement pluck;
And yet methinks I have Astronomy,
But not to tell of good or evil luck,
Of plagues, of dearths, or seasons' quality;
— William Shakespeare, Sonnet 14, 1609

Terms like opposition or superior conjunction or greatest elongation west often appear in the pages at SkyEye. These words describe certain special placements of the planets relative to the Sun and Earth. The accompanying diagram illustrates some of the more interesting configurations.

The inferior planets Mercury and Venus move around the Sun within the orbit of the Earth. Because they are closer to the Sun than the Earth, they move around their orbits more quickly, completing a complete revolution in under a year.

Inferior Conjunction

When an inferior planet passes through the line between the Sun and Earth, that planet is said to be at inferior conjunction. Because their orbits are inclined at a slight angle to the ecliptic, Mercury and Venus normally pass slightly north or south of the Sun's disk. They are not visible at this time as they are hidden in the Sun's glare. However, occasionally an inferior planet reaches an orbital node (the place where the orbit crosses the ecliptic) at the same time it arrives at inferior conjunction. In this case, the planet may be seen (with appropriate optical aids) gliding across the face of the Sun. Transits of Mercury occur about 12 or 13 times a century. Transits of Venus are far more rare, happening approximately four times every 243 years, with pairs of transits eight years apart and separated by long gaps. The most recent transits of Venus were in 2004 and 2012, and the next one will not occur until 2117.

Greatest Elongation West

As it moves around the Sun, an inferior planet quickly forms a right triangle with the Sun and Earth and is said to be at greatest elongation west. A planet at greatest elongation west is visible in the morning sky before sunrise. As the name suggests, the planet is at its maximum angular distance from the Sun as seen from Earth.

Superior Conjunction

The angular distance between the inferior planet and the Sun decreases until the planet is exactly in line with our star. This instant is called superior conjunction. As with inferior conjunction, the planet usually passes to the north or the south of the Sun's disk and is impossible to view. However, if the inferior planet is near a node at the time of superior conjunction, it may pass behind the Sun in an event sometimes called an anti-transit or secondary eclipse. It is not observable from Earth because of the proximity of the Sun.

Greatest Elongation East

Now an evening sky object visible in the west after sunset, the inferior planet slowly moves away from the Sun until it reaches its maximum angular distance away from the star. This is called greatest elongation east. After attaining this position, the inferior planet closes in on the Sun and its next inferior conjunction.

The superior planets (Mars, Jupiter, Saturn, Uranus and Neptune) go around the Sun beyond Earth's orbit. As a result, their orbital speeds are smaller than that of Earth. The further from the Sun you go, the slower the speed.

Opposition

Opposition occurs when the superior planet is opposite to the Sun in the sky. It rises around sunset, passes the meridian around local midnight and sets around sunrise. Superior planets are at their nearest and brightest around opposition, often on the same day. Mars is the noticable exception. Its orbit is more eccentric than those of the other superior planets, and opposition and closest approach to Earth may differ by over a week.

East Quadrature

Quadrature occurs when the Earth is at a right angle between the Sun and superior planet. Mars can look distinctly gibbous (more than half illuminated but less than full) around this time but the gas giants don't look any different than they usually do. The shadows cast by the planets and their satellites (and its rings in the case of Saturn) are a little off to one side, leading to some interesting astrophotographic opportunities. A superior planet at east quadrature sets around local midnight, making it an evening sky object.

Conjunction

Conjunction of a superior planet is the same as superior conjunction of an inferior planet, in that the object is on the far side of the Sun from Earth. It is lost to view for days or weeks before and after conjunction. As with inferior planets, if a superior planet is near a node at the time of conjunction, it might pass behind the disk of the Sun as seen from Earth. Normally, however, the superior planet passes to the north or south of the Sun.

West Quadrature

A superior planet at west quadrature rises around local midnight and is visible in the morning hours.