Elongation is the angular distance between a planet (or other solar system body) and the Sun, as seen from Earth. It is the single most important number for determining whether a planet is observable on a given night and where in the sky to look for it. What It Is Elongation is measured in degrees, from 0° to 180°. An elongation of 0° means the planet is in the same direction as the Sun (conjunction) — invisible, lost in solar glare.

Perihelion is the point in a planet’s (or comet’s or spacecraft’s) orbit at which it is closest to the Sun. Aphelion is the point at which it is farthest. The two terms define the extremes of an elliptical orbit, and their existence explains why Earth’s seasons are not driven by distance from the Sun. What They Are Planetary orbits are ellipses, not circles. Johannes Kepler established this in his first law of planetary motion (1609): every planet moves in an ellipse with the Sun at one focus.

Syzygy is the astronomical term for the alignment of three or more celestial bodies in a gravitational system along a straight line. Solar eclipses, lunar eclipses, and the extreme tides called spring tides all result from syzygy. The word is rare enough to stop most readers; the phenomenon it describes happens every month. What It Is In the Earth-Moon-Sun system, syzygy occurs at two points in the lunar cycle. At new moon, the Moon lies between Earth and the Sun — the three bodies align with the Moon in the middle.

The ecliptic is the apparent path of the Sun across the celestial sphere over the course of a year, as seen from Earth. It is also the plane of Earth’s orbit around the Sun. The zodiac constellations lie along the ecliptic. Eclipses happen only near it. And the word itself tells you why. What It Is As Earth orbits the Sun, the Sun appears — from Earth’s perspective — to move against the background of stars, tracing a great circle across the sky over twelve months.