I may reference this blog in the future alot.
How does one find ones way in the night sky? In ancient times constellations were the landmarks in the sky. The agrarian revolution led to the invention of more precise systems of angular measurement. Astrologers could predict the motion of the stars and planets. The invention of calendars made it possible to predict the seasons. To ancient people the math seemed like spell casting and divination and the astrologers rudimentary meteorological skills seemed like downright prophecy.
Throughout this blog I will mostly use decimal degrees or degrees minutes seconds (°, ', "). I will sometimes use time units as a sort of angular measurement (described below). When I post computer programing code, for reasons that are beyond the scope of this post, I will often use radians.
Most celestial coordinate systems are a variation of spherical coordinates. If you are lucky enough to have taken calculus you know that spherical coordinates are described with a distance and two angles. Though many star catalogs give all three values as well as other data, were are mostly concerned with the two angles. In vector terminology we are interested in direction but not magnitude.
I will refer to an object. This can be anything, such as a star, planet, spacecraft or Santa's sleigh.
Topocentric Coordinates
The first system of coordinates is the topocentric coordinates. You may also here this system called horizon or altazimuth coordinates. This system is centered around the observer. The z-axis corresponds to straight from the ground. The zenith is the highest point in the sky. The angular separation between the object and the virtual horizon (where the horizon would be if the landscape were totally flat) is called the objects altitude. The heading of the object given in degrees clockwise from true north is called its azimuth.
The advantage of this system is that it allows quick observations without needing to refer to other constellations just to know where to look. It is popular among amateur astronomers. The disadvantage is that in this coordinate system everything is moving all the time except you guide star (Polaris for the north hemisphere) and they are totally specific to your particular geographic location (your terrestrial coordinates, latitude and longitude).
Equatorial Coordinates
The angel the object makes with the equator is know as declination. Declination is positive for angles above the equatorial plane and negative for angles below. Polaris has a declination of 90°.
Right ascension is usually measured in hours, minutes, and seconds east of the first point of Aries. The first point of Aries is where the sun crosses the equator each year during the vernal equinox.
Azimuth telescope mounts are good for finding objects with topocentric coordinates. Equatorial mounts are good for finding objects with equatorial coordinates and tracking objects as they arc across the sky. For high magnification telescopes or astrophotography you will want an equatorial mount with a clock drive. A clock drive will move the telescope at exactly the rate of rotation of the earth along the axis parallel to the earth's rotation.
Precession and the Ecliptic
Equatorial coordinates seem fine and dandy until one day, say two thousand years from now, you realize that Polaris simply doesn't point due north anymore. The process is known as precession. The earth always maintains an axial tilt of 23.4° but it tends to wobble like a top. Geographic north moves through a complete circle every 26,000 years. The equinox or solstice occurs at different times of year throughout the precession. Star catalogs, which publish data in equatorial coordinates, are only accurate for the published epoch.
In ancient times the vernal equinox happened in the constellation Aries. The summer solstice happened in Cancer. The winter solstice happened in Capricorn. Now thanks to precession names like the Point of Aries, tropic of Cancer and tropic of Capricorn just don't make any sense anymore.
The solution is to use a coordinate system that will be rock steady for the next four and a half billion years. I am talking about the plane of the ecliptic. The plane of the ecliptic is the plane that the earths orbit around the sun inscribes. Coordinates are given in ecliptic latitude and ecliptic longitude. Unfortunately the individuals who devised this system chose to place the zero point at the vernal equinox so this coordinate system is still subject to the affects of precession.
The ecliptic is useful in observing planetary bodies as most inscribe paths across the sky that are very close to the ecliptic.
| Abreviation | Name |
| a | Altitude |
| A | Azimuth |
| dec or δ | Declination |
| R.A. or α | Right Ascension |
| H | Hour Angle |
| β | Ecliptic Latitude |
| λ | Ecliptic Longitude |
Time
Time as is used everyday is known as solar time. A 24 hour day is the amount of time it takes the earth to rotate about 361° degrees. The extra degree is the angular quantity the earth revolved around the sun in one day.
Astronomers sometimes prefer a quantity called sidereal time. One sidereal day is the time it takes the earth to rotate exactly 360°. One sidereal second is exactly 15 seconds of arc.
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