Points of reference

¡SkyCaramba! Weekly astronomy blog for the week ending June 28, 2014

If you’re an astronomer who’s going to find your way around the sky or describe where you saw something, you need to know some important points of reference.

I don’t suppose I have to tell you what is meant by north, south, east, and west. If you know an object will be in the east, you look in the direction the sun and moon rise in. It’s probably worth noting, however, that when an astronomer says an object is moving northward, he or she usually means it’s moving toward the north celestial pole. That may not be due north or north at all compared to objects on the earth.

If you wish to specify than an object is overhead, it is at the zenith. And if it’s beneath your feet on the other side of the earth, it’s at the nadir. The horizon is that line that makes a big circle around you between the earth and sky. Realistically, your horizon is irregular because of the terrain or ocean waves rising and falling. For astronomy purposes, it can be thought of as separating everything in the dome above from the dome below whether or not buildings and mountains are in the way.

Azimuth converts the cardinal directions into degrees of a circle. North is either 0° or 360°. East is 90°, south is 180°, and west is 270°. An object at azimuth 45° would be in the northeast. Something at 195° would be a little west of due south. Usually, if an astronomy math program calculates an azimuth value outside the range of 0 to 360, it adds or subtracts 360 until the result is within range. But if you ever encounter one outside the range, you can make the correction yourself.

Altitude refers to how high an object is on the celestial dome. If it’s at 90° altitude, it’s directly overhead. At 0° it’s on the horizon. You see first light at dawn when the sun has about -15° altitude, in other words 15° below the horizon.

The meridian is the line that runs overhead from north to south. The word could also refer to lines of longitude. If you need to make a distinction, the one that’s over your head is the local meridian.

The anti-meridian is the line running from north to south on the opposite side of the earth. Sometimes, an astronomer calculates when the sun is on the anti-meridian and when a planet is on the meridian at the same time. The planet would be at opposition then.

We use lines of longitude and latitude to determine coordinates on Earth. In the sky, we can use celestial longitude and celestial latitude. However, they are skewed compared to the terrestrial lines. Celestial latitude lines are parallel to the ecliptic, the line the sun travels along. Likewise, celestial longitude lines are at right angles to the ecliptic. This system of marking astronomical positions is called the ecliptic system.

The celestial equator is over the earth’s equator. The coordinate lines running parallel to it are called declination. Those at right angles to it are called right ascension. Declination and right ascension are most akin to the terrestrial versions of latitude and longitude. This is the equatorial system of marking positions.

A planet that’s moving eastward is in direct motion. Westward is called retrograde motion.

That should help you understand a little more when you read about how to find things in the sky.