As you look up at the night sky and observe how the Sun, Moon and planets move across it, they tend to appear close to an invisible line known as the ecliptic. This invisible line marks Earth’s orbit upon the celestial sphere – it also serves as the basis of an astronomer-approved celestial coordinate system used to precisely locate stars, nebulae and galaxies.
All planets in our Solar System – asteroids and comets included – follow heliocentric orbits around the Sun. This means they orbit in a plane at a constant speed over an equal period. Furthermore, planets move faster when nearer the Sun but slow as they get further away; their closest approach point being called their periapsis while its farthest away point its apoapsis; also note the Sun crosses its own orbit twice every year at points called vernal and autumnal equinoxes!
The Ecliptic lies at the center of a region in the sky known as the Zodiac, which spans its surface like a great circle. This area has traditionally been divided into 12 signs with 30 degree longitude intervals between them, which correspond to constellations located along that section of ecliptic. Astrologers believe each constellation lies under its respective sign has an impactful impact upon planets or stars within that sign’s realm.
Deliberation on whether the signs of the zodiac are accurate astrologically is controversial; nonetheless they serve as an useful way of tracking the Sun and other planets as they move across the sky. They’re also useful as starting points for various astronomical observations performed on both earth and moon.
Lunar eclipses occur when the Moon crosses over the ecliptic. When this happens on either an annular or penumbral eclipse date (equinox, apoapsis of ecliptic), respectively.
This photo, captured by the 1994 Lunar Prospecting Clementine satellite, depicts well the plane of the ecliptic. From right to left it shows Earthshine reflecting off of the Moon; Suncorona rising over dark limb; Saturn, Mars and Mercury planets visible against dark backdrop of sky; etc. Hubble Space Telescope’s stunning image clearly displays the ecliptic plane, showing Venus and Mars moving in their orbit around our Sun, as well as six other planets located close to it. As planets were all created from the Sun’s spinning, flattened proto-planetary disk, keeping an eye on the sky during summer months is worth keeping an eye on to trace its ecliptic path as Sun, Moon and planets pass above or below it. Soon enough you will become acquainted with its pattern; and be able to recognize it on clear nights in stars, nebulae and constellations visible therein.