Altitude-Azimuth
The altitude-azimuth method can be used to describe the location of an object in the sky, usually a celestial object, from the observer. To do this, the altitude and the azimuth of the object must be measured. The altitude is the angle between a horizontal line pointing towards the horizon (at sea level) and a straight line in the direction of the object. The azimuth is the angular distance of the object from a zero point, usually north, along the horizon.
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Right ASCENSION-Declination
Right ascension-declination is used to describe the position of stars on the celestial sphere in the same way that we use longitude and latitude on earth. Declination can be seen as a continuation of latitude. For example, above the Earth's equator is a declination of 0 degrees, above the Earth's North Pole is the north celestial pole with a declination of 90 degrees and the Southern Cross is positioned on declination 60 degrees.
Right ascension (RA) relates the height of stars in the sky to time. It was given its name, as when facing north, the stars appear to ascend on your right. RA is usually measured in hours, which means the sky can be dived into 24 sections, where one section equals 15 degrees or one hour. The point where right ascension equals zero is the vernal equinox. The right ascension of a star is almost the same as the time between when it crosses the Greenwich meridian and when the vernal equinox crosses it.
Right ascension (RA) relates the height of stars in the sky to time. It was given its name, as when facing north, the stars appear to ascend on your right. RA is usually measured in hours, which means the sky can be dived into 24 sections, where one section equals 15 degrees or one hour. The point where right ascension equals zero is the vernal equinox. The right ascension of a star is almost the same as the time between when it crosses the Greenwich meridian and when the vernal equinox crosses it.
Diurnal and Annual Motion
The diurnal motion of the sky is the daily motion of the planets and stars moving around the celestial sphere. The celestial sphere is based on the Ptolemaic system, in which everything revolves around Earth. This means that every day, when the Earth shifts one degree east, it is seen as the stars shifting one degree west.One diurnal rotation takes 23 hours 56 minutes and 4.09 seconds. Stars that are close to the poles are circumpolar, meaning they never disappear below the horizon. There are also stars, that from the opposite hemisphere, you will never see rise. For example, the star Polaris, which is close to the NCP, remains out of site when in Australia. Equatorial stars rise and set every night as they are near the equator.
The annual motion of the sky describes the yearly patterns the stars make through space. Due to the Earth's rotation around the Sun, some stars rise and set at different times throughout the year. Over the year, the Sun slowly drifts to the East. This movement is less than one degree each day, as there are 365 days in a year and only 360 degrees in a circle. The Sun travels on a path called the ecliptic (see image above right), which is tilted 23.5 degrees from the equator. The points where it crosses the equator are the equinoxes. These represent the beginning of autumn and spring for the Southern and Northern hemispheres as it is where the Sun crosses in and out of each hemisphere. The planets also move eastward through the sky, this is called prograde motion. However, sometimes they move in a westerly direction in relation to the stars, this is called retrograde motion.
The annual motion of the sky describes the yearly patterns the stars make through space. Due to the Earth's rotation around the Sun, some stars rise and set at different times throughout the year. Over the year, the Sun slowly drifts to the East. This movement is less than one degree each day, as there are 365 days in a year and only 360 degrees in a circle. The Sun travels on a path called the ecliptic (see image above right), which is tilted 23.5 degrees from the equator. The points where it crosses the equator are the equinoxes. These represent the beginning of autumn and spring for the Southern and Northern hemispheres as it is where the Sun crosses in and out of each hemisphere. The planets also move eastward through the sky, this is called prograde motion. However, sometimes they move in a westerly direction in relation to the stars, this is called retrograde motion.