Parallax, Precession, and the Crucial Angles
How can you tell a friend exactly where to find a star besides pointing at it? Well, it’s a bit complicated, but then what isn’t about the Earth?
Astronomers once used the constellations to help them say, for instance, that Altair is in the constellation Aquila. Then they needed to be more precise. Using a system analogous to the geographic survey points on Earth, they located the stars by points north-south and east-west in the sky.
If you’re a boater, you probably know that Bainbridge lies between 122º 28’ and 122º 36’W and 47º 34’ and 47º 43.5’N, or about 122 degrees west of Greenwich, England and 47 degrees 37 minutes north of the equator. If you are setting up a telescope, you know your location down to the mini-seconds of the degrees. Similarly the location for a star, bright Altair for instance, is at RA 19h 50.47m DEC +08º 52’.
What does this cryptic language for the star mean?
The lines that divide the Earth east and west we call longitude. Longitude is a convention that established the starting point at the Royal Observatory in Greenwich, England in 1884. In the sky, the base line runs in a great circle from due north through the point and time at which the Sun has crossed the celestial equator at the spring equinox to the celestial south pole, through the point of the fall equinox, and back to due north. Astronomers call this analogue of longitude right ascension (RA). The point of the vernal equinox is noted as RA 0h 0.0m; the fall equinox is at RA 12h 0.0m. The celestial sphere is divided east-west into twenty-four hour segments, and that’s subdivided into minutes and seconds. (Because the Earth’s axis on which it rotates moves (precesses) very slowly, RA 0h is not now the exact point of the spring equinox. Astronomers check the ephemera listing for a star’s current location.
The line that divides the Earth in half north and south we call the equator. The lines parallel to it north and south indicate latitude. Astronomers have projected in imagination into the sky the same line directly overhead above the equator; this is what the Sun traces on the days in spring and fall when the Sun rises due east and sets due west. This line is called the celestial equator. In astronomical terms latitude is identified as declination (DEC), or the base line to which anything that “declines” north or south of the celestial equator is related. It divides north and south into 90 degrees with the North Pole at DEC 90º 0.0’N, or +90º.
Star locations are given by the places in the sky where these imaginary lines of right ascension (RA) and declination (DEC) cross. These are the exact arrangements, or coordinates, of the stars.
Right ascension is usually given in hours (h) and minutes (m). The distance is divided this way because the Earth’s circumference can be measured in 360 degrees, and because the sky turns around us every 24 hours. Each hour, or one hour of right ascension, is equal to 15 degrees in that turning.
Declination is measured in degrees (º) and minutes ('); south is often indicated with a “-” sign, north with a “+” sign. Relating this to the coordinates for Altair, the star is at RA 19h 50.47m DEC 08º 52’. Another way of saying this is that Altair rises almost 20 hours after the point of the spring equinox.
In the next issue we’ll say something about another celestial line, the ecliptic.