Space Science
There are two different coordinate systems to locate stars in the sky:
1. Right Ascension-Declination
• Vernal equinox is where the ecliptic and celestial equator meet.
• Ecliptic is the apparent path of the Sun in one year (it is also along the plane in which the Earth revolves around the Sun).
• Right ascension is analogous to longitude and is measured in hours, minutes, and seconds, from the point of vernal equinox.
• Declination is analogous to latitude and is measured in degrees from the celestial equator.
• Measurements of declination are either positive (north) or negative (south).
• Note: 1 hour = 15° of arc (1° arc is equivalent 1° of longitude) and 1 minute = 1/60° of arc.
• Example: Proxima Centauri has a declination of -62°40’ (the negative indicates it is located 62° below the celestial equator) and a right ascension of 14h 29.7m.
2. Altitude-Azimuth
• Altitude is the angular distance, measured from 0 to 90 degrees, of a celestial object above the observer’s horizon. Azimuth is the compass angle from due N to the location of the celestial object.
Astronomers prefer the right ascension-declination coordinate system because these values remain unchanged over time, whereas the altitude and azimuth values will be different from one location to another, and they will also change with time. For example, as a star rises in the east it will have a low altitude angle, as it rises in the sky the angle from the horizon increases as does the azimuth direction.
Circumpolar stars, Asterisms, and Constellations
• Stars that around the north celestial pole are called circumpolar stars.
• An area in the sky defines a constellation.
• An asterism is a group of recognizable stars that is not recognized as an official constellation. A constellation will often contain an asterism within its area, for example the big dipper is a part of the constellation Ursa Major.
• Although some of the circumpolar constellations (for example Ursa Major and Ursa Minor)
1. Right Ascension-Declination
• Vernal equinox is where the ecliptic and celestial equator meet.
• Ecliptic is the apparent path of the Sun in one year (it is also along the plane in which the Earth revolves around the Sun).
• Right ascension is analogous to longitude and is measured in hours, minutes, and seconds, from the point of vernal equinox.
• Declination is analogous to latitude and is measured in degrees from the celestial equator.
• Measurements of declination are either positive (north) or negative (south).
• Note: 1 hour = 15° of arc (1° arc is equivalent 1° of longitude) and 1 minute = 1/60° of arc.
• Example: Proxima Centauri has a declination of -62°40’ (the negative indicates it is located 62° below the celestial equator) and a right ascension of 14h 29.7m.
2. Altitude-Azimuth
• Altitude is the angular distance, measured from 0 to 90 degrees, of a celestial object above the observer’s horizon. Azimuth is the compass angle from due N to the location of the celestial object.
Astronomers prefer the right ascension-declination coordinate system because these values remain unchanged over time, whereas the altitude and azimuth values will be different from one location to another, and they will also change with time. For example, as a star rises in the east it will have a low altitude angle, as it rises in the sky the angle from the horizon increases as does the azimuth direction.
Circumpolar stars, Asterisms, and Constellations
• Stars that around the north celestial pole are called circumpolar stars.
• An area in the sky defines a constellation.
• An asterism is a group of recognizable stars that is not recognized as an official constellation. A constellation will often contain an asterism within its area, for example the big dipper is a part of the constellation Ursa Major.
• Although some of the circumpolar constellations (for example Ursa Major and Ursa Minor)