Ecliptic
By Anna Edmonds
In the July-August Newsletter we talked about the coordinates of a star, its right ascension (RA) and declination (Dec.). The path of the ecliptic is another measure that’s useful in finding the locations of things in the sky, particularly the planets.
To recap, right ascension marks a distance east or west from the imaginary line that starts on the vernal equinox directly above the Royal Observatory in Greenwich, England. Declination likewise is an imaginary line; its base is directly above the Earth’s equator, and the measurements describe the distance north or south from it. The Earth’s equator is projected into the sky to describe the celestial equator.
The ecliptic is the path of the Earth’s orbit around the Sun. The Earth spins itself around once every day; we see the effect of this rotation with the Sun rising every morning and setting every evening.
But the Earth’s rotation is n’t perpendicular to the Sun; it’s tilted at a 23 ½º angle as it revolves around the Sun every year.
The tilting takes the plane of the Earth’s revolution above the plane of the celestial equator for half the year, and below it the other half. The two planes intersect at the points of the spring and the fall equinoxes. At these times the Sun rises due east and sets due west, and the day time is equal to the night time. Without this yearly swinging the Sun would always rise and set due east and west. The tilting carries the Earth farthest north of the celestial equator at our Northern Hemisphere summer solstice, and farthest south at our winter solstice. It also means that the Earth experiences the changing seasons—and for life here this is critical. For us in winter the Sun’s rays fall at a low angle when the Sun appears in the southern sky. Its path then above the horizon is short, making the daylight hours short. Six months later the Sun is to the north; its rays come at us from higher in the sky, and the northern half of the Earth experiences summer.
The ecliptic is also the path that the Sun appears to follow every year around the stars as we look at the sky from Earth. It, too, is at the same tilt of 23 ½º. As we look towards the Sun over the course of the year, because of our revolution the Sun appears to move around against the background of the constellations. Ancient astronomers observed this regular path and named the groupings of stars that the Sun moves through each month the zodiac.
The orbit of the Moon is at a 5º tilt to that of the Earth’s orbit. Half the time the Moon is above our orbit, half below. When the Moon comes close to our ecliptic, that is, when the Sun, the Moon, and the Earth are in a line with the Moon between the others, then the Moon’s shadow can hit the Earth, and the Sun is eclipsed. Similarly, when the Earth is between the Sun and the Moon the Moon will be in eclipse. At these times the Moon is in the same plane that we are—the ecliptic.
All of the planets, except for odd Pluto, revolve around the Sun in almost the same plane that we do. The planets are always located close to the path of the Sun and within the zodiac. Since the Moon moves within that path, if you can see the Moon and a planet, you can use the arc that they mark to look for any other planets that may be visible.