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CELESTIAL SPHERE
In ancient times, the night sky was a blaze of stars, not the light polluted skies we now have. Observers were quick to note the patterns of the stars repeated each night changing gradually with the seasons. They decided that the sky was in fact a large black sphere, with holes in it, behind which were placed lamps. The concept of the sphere has stuck, but our understanding of the nature of the lights has changed our view of the universe totally.
Coordinate Systems
It's not a bad model or paradigm to use for mapping the night sky, because the stars appear to stay put from one year to the next and go round us, but there are a couple of interesting points we notice right away. The stars appear to revolve around a point in the sky once a day that is above the North Pole, this is due to the rotation of the Earth. The point above the North Pole is known as the North Celestial Pole (NCP), there is also a South Celestial Pole (SCP), but we can't see it from the UK.
AltAz
We tend to refer to locations on a map in terms of Latitude and Longitude, when making measurements of objects on Earth or in the sky (terrestrial observation) we would specify their location with the terms Altitude (degrees above the horizon) and Azimuth (degrees from true North). However, when looking at the night sky, we observe that the background stars are moving with respect to Earth. In fact, in terms of AltAZ, they move at 1 degree per 4 minutes or 15 degrees an hour (sidereal rate or 'star time').
Declination
If we mark the NCP as 90 degree and mark the Celestial Sphere downwards in degree increments (termed the Declination) to 0 degrees, we find the Celestial Equator sits above the Earth's Equator. However, we notice that the Sun does not travel along the line of the Equator, instead it is above or below the Equator, only passing directly over it twice a year. The Earth's tilt on its axis is termed its obliquity. We call the path that the Sun takes around the Celestial Sphere the 'Ecliptic'. If we were to mark the Suns position against the background stars at the same time each day, for a year, we would see that the Sun's path through the stars follows a curve. The Ecliptic is curved because the Earth is tilted on its axis as it orbits the Sun. The angle of tilt is 23.5 degrees, this is what gives rise to our seasons (because in summer, the Sun is above the Northern half of the Celestial Sphere giving longer days).
Equinox
When the Sun crosses the Equator, the length of the day is the same as the night (equal night - 'Equi-nox'). This happens twice a year, in spring - the Vernal Equinox, and again in autumn. Because the Earth is orbiting around the Sun, the background stars appear to move each night. In reality it is the position of Earth that is moving with respect to the Sun. The amount of movement is slightly larger than 1 degree per day. In order to measure or describe on a chart where the stars are, we use the Vernal (spring) Equinox as our starting point; this is the point on a chart that the Sun is at during the Equinox.
Right Ascension
Although not a literal point, but a calculated point, close to the Vernal Equinox, to base our charts and calculations on (you have to start somewhere), we have a point we call 0 or Zero Hour. The Earth rotates on its axis once every 23 hours 56 minutes and 4 seconds if you measure it using the stars (sidereal time). However, the Sun returns to the same position in the sky once every 24 hours on average (solar time) the difference is because the Sun's relative position has moved because we are orbiting the Sun. On star charts therefore we use 24 hours as the measure of distance along (East to West), or hours of Right Ascension.
The Zodiac
The Sun's track through the stars takes it through the constellations of the Zodiac once a year, for convenience, early observers referred to 12 constellations in the Zodiac (to match the number of months) but in fact there are 13. The objects in the night sky that were first discovered to move (the planets and the Moon) and which eclipse and occult each other (block the light) occur within a narrow band of the plane of the ecliptic. Solar and Lunar eclipses occur only on it.
Diagram of the night sky, depicting stars, the ecliptic, the equator, and lines of right ascension and declination.
Terms
When we observe a star on the horizon in the East, it appears to rise in azimuth up the night sky and set in the West. At its highest point, it is said to be at its Zenith. When a star reaches its lowest point, it is said to be at its Nadir. As the star reaches its peak it is said to have reached its culmination.
If we were to draw an arc through a star at it's zenith on the celestial sphere joining the NCP to the SCP, we would have created a local Meridian for that star (and for the observer). The Sun reaches its zenith on midday on the longest day of the year at the summer solstice. The latitude of this point on the Earth's surface immediately under the Sun at which this occurs (23.5 degrees) is known as a Tropic. Since the Sun was in the zodiacal constellation of Cancer at the time it was named for the Northern Hemisphere, it is known as the Tropic of Cancer (the tropic of Capricorn being the most southerly extreme of the Sun's zeniths, and our mid-winter).
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