1. How do we locate objects in the sky? Angular separation: the angle between objects as seen from Earth (Degrees are separated into minutes (60 minutes of arc /degree) and minutes are divided into 60 seconds of arc). You can also just use decimals to denote partial degrees. Think: Does the angular separation of 2 objects in the sky tell us how far apart they are? The reference frame is very important in locating objects in the sky. Our BH/LH method measures locations relative to the observer. The altitude is the vertical component—how far above the horizon (the 0 point) the object is. The zenith altitude is 90 degrees. The name for an altitude of -90 degrees (or 180) is nadir. The azimuth is the horizontal component—how far away from North the object is. The direction “North” is our 0 reference level.

2. Altitude/Azimuth Schematic

3. What if we need to locate an object relative to another reference point? Relative to the equator of the Earth, the terms declination and right ascension are used. Declination refers to the vertical component and right ascension refers to the horizontal. Declination corresponds to latitude on Earth’s surface while right ascension corresponds to longitude. The celestial equator is the 0 degree vertical reference and the vernal equinox is the horizontal 0 reference point. This point at the intersection of the ecliptic and the celestial equator. The sun is located at this point on the vernal equinox. (An analogous point, the autumnal equinox, is located on the opposite side of the celestial sphere).

5. We could also locate objects relative to the ecliptic, which is the Sun’s apparent orbit projected onto the celestial sphere. We will talk more about the celestial sphere in upcoming classes, but for now just think of it as a model for the observable sky. The difference between using the ecliptic and using the celestial equator as our reference is a matter of degree—23.4 degrees to be exact, since that is how much the Earth is tilted.

7. What exactly is an equinox? A day or a point on the celestial sphere… The vernal equinox is the day the Sun is located where the ecliptic intersects the celestial equator moving South to North; also refers to that location on the celestial sphere. The autumnal equinox is the day the Sun is located on the ecliptic 180 ⁰ from the vernal equinox; also that location on the celestial sphere. On the equinoxes, day and night are approximately equal.

8. What is a solstice? Either of the two times in the year, the June solstice and the December solstice, when the sun reaches its highest or lowest point in the sky, marked by the longest and shortest days of the year. Also referred to as summer and winter solstices, but this is ambiguous as summer and winter depend upon your location on Earth.