Distant Nature: Astronomy Exercises
Objectives
There are two primary coordinate systems used in astronomy for locating celestial objects as viewed from Earth. These are the Altitude/Azimuth coordinate system and the Right ascension / Declination coordinate system. In this exercise, you will learn about the use of each of these systems and the pros and cons of each.
Equipment/Materials Needed
Stellarium
Subject Introduction
Altitude/Azimuth (Alt/Az) Coordinates
The Alt/Az coordinate system is used to describe the location of an object with the angle above the level horizon (Altitude) and the direction (angle) of view along the level horizon (Azimuth). The Altitude angle is measured from the horizon upward. Looking at the horizon is 0°, straight overhead is 90° (the Zenith). Looking straight down is -90° (the Nadir, directly opposite the Zenith). The azimuth angle is usually measured clockwise from due North, where:
- North is 0°
- East is 90°
- South is 180°
- West is 270°
The Azimuth is sometimes stated in terms of the relative clockwise angle from one of the compass cardinal points, but we will not use this method.
The Altitude/Azimuth coordinate is a simple system that was even used by the ancients in navigation. It can be used to very simply describe the location of an object in the sky, but its value is limited in that it is different for different times and different locations on Earth. See Figure 1.
Alt/Az Coordinates
Figure 1.
Right Ascension/Declination (RA/Dec) Coordinates
The Right Ascension/Declination (RA/Dec) coordinate system also uses two angles to describe a location in the sky. Instead of being measured from North and from your local level horizon, these are measured against standardized points on the celestial sphere. This is the imaginary sphere surrounding the Earth with the celestial poles directly above/below the Earth's poles and the celestial equator directly above the Earth;s equator. The RA angle is measured from the Vernal Equinox and is measured in units of Hours, Minutes and Seconds. (hms). The complete circle in RA is 24h. The Dec angle mimics Latitude on Earth, going from 0° on the celestial equator to 90° at the North celestial pole (NCP) and -90° at the South celestial pole (SCP). See Figure 2.
RA/Dec coordinates do not change with the observer's location on Earth and will not change over time (at least not noticeable). For this reason, this system is the one used most often by astronomers in star charts and object catalogs.
RA/Dec Coordinates
Figure 2.