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Download 3.2a Right Ascension and Declination
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3.2a demonstrate an understanding of the terms ‘right ascension’ and declination When we look at a map, we locate the position of a place using latitude and longitude. If we look out at the night sky and imagine the stars are painted on a sphere around the Earth, we locate a particular celestial object in a similar way using declination (dec) and right ascension (RA). Comparing the Earth with the night sky:declination (dec) is equivalent to latitude right ascension (RA) is equivalent to longitude Declination (dec) Declination is the angular distance north or south of the celestial equator Declination North of the celestial equator is positive +90˚ = North Celestial Pole NORTH CELESTIAL POLE CELESTIAL EQUATOR Declination South of the celestial equator is negative -90˚ = South Celestial Pole SOUTH CELESTIAL POLE Picture credit : (Earth) NASA’s Earth Observatory We need reference points for latitude or declination from which to measure:Reference point EARTH NIGHT SKY VERTICAL EQUATOR Latitude CELESTIAL EQUATOR Declination In the diagram below, looking out from the Earth, if we imagine the line of our equator extended into space, we get the celestial equator. Instead of latitude, astronomers use the word declination. The equator is given as 0˚. Whereas latitude may be 30˚N, anything north of the celestial equator is plus = dec +30˚. Anything south of the celestial equator is minus = dec -30˚ (equivalent to latitude 30˚S). DECLINATION (degrees) +30˚ 0˚ CELESTIAL EQUATOR -30˚ Above the North Pole is dec +90˚ and above the South Pole is dec -90˚ Picture credit : (Earth) NASA/JSC-Apollo17 For the purposes of this explanation, the celestial sphere has been reduced. A person has the sky curving round behind them – the view above shows the full band of stars between declination +30˚ and -30˚. Right Ascension (RA) If a person was able to see the night sky shown above for a full day, the full band of stars would pass in front of them, moving steadily towards the right. The longitude reference point was more difficult. Many countries have laid claim to the Prime Meridian – the Chinese once used a gate from the Forbidden City to Tiananmen Square to mark the line. The GREENWICH PRIME MERIDIAN was chosen as the 0˚ reference point in 1884. The scale of Right Ascension is like a clock. The Earth spins every 24 hours (actually every 23 hours 56 minutes) and the clock was used to divide the night sky. Every hour, another 15˚ of the night sky passes an observer. Just as with a clock, parts of an hour were divided into minutes. Reference point EARTH NIGHT SKY HORIZONTAL GREENWICH MERIDIAN Longitude VERNAL/SPRING EQUINOX Right Ascension GREENWICH PRIME MERIDIAN 360˚ 0˚ Yours truly standing at the Royal Observatory in Greenwich with my right foot in the east and my left foot in the west! The PRIME MERIDIAN is the line between my feet – the reference line for LONGITUDE for the whole world. As there are 360˚ in a circle, the circle around the world could be referenced – a location could be given in degrees eg on the opposite side of the world to Greenwich, the longitude is 180˚. On the celestial sphere, half way round the sphere is 12 hours 0 minutes, which is the value of Right Ascension for a star at that point. The night sky moves across the sky as shown by the arrows in the diagram (ANGLED AT 23.5˚ because of the tilt of the Earth’s axis) Autumn Equinox Right Ascension = 12 hr 0 min (half way round the circle) NORTH CELESTIAL POLE Summer Solstice Right Ascension = 6 hr 0 min (a quarter of the way round the circle) CELESTIAL EQUATOR 23.5˚ Winter Solstice Right Ascension = 18 hr 0 min (three quarters of the way round the circle) RA up to 24hr 0min SOUTH CELESTIAL POLE Spring Equinox Right Ascension = 0 hr 0 min (the starting point of measurements) Picture credit : (Earth) NASA’s Earth Observatory The key part for measuring Right Ascension is when to start the clock. The decision was taken to choose the Spring Equinox – when the path of the Sun cuts the celestial equator moving northwards. Having decided on this reference point, an astronomer situated at the Prime Meridian in Greenwich just needed to observe the stars at that moment. Once the stars on the line at 0 hours 0 minutes had been recorded, the rest of the night sky could be catalogued by careful observation. One hour later, stars on the Prime Meridian line were given a Right Ascension value of 1 hour 0 minutes. Values for Right Ascension for a number of stars are shown in the following chart:Star name Constellation Right Ascension (RA) Polaris Ursa Minor 2 hours 32 minutes Sirius Canis Major 6 hours 45 minutes Regulus Leo 10 hours 08 minutes Deneb Cygnus 20 hours 42 minutes During 24 hours, the Earth makes a full rotation through 360˚ (15˚ per hour). The stars drift across the night sky from East to West. In the diagram below, vertical lines are shown for every 2 hours on the portion of the night sky between about dec +30˚ and -30˚. +30˚ 0˚ -30˚ 24 22 20 18 16 14 12 10 8 6 4 2 0 22 20 18 16 RIGHT ASCENSION (HOURS) DRIFT OF STARS ACROSS THE NIGHT SKY +30˚ 0˚ -30˚ 16 14 12 10 8 6 4 2 24 /0 RIGHT ASCENSION (HOURS) 16 HOURS LATER, THE STARS HAVE DRIFTED ACROSS THE NIGHT SKY The reference point for the night sky is the point at which the Sun crosses the celestial equator moving North (the Vernal/Spring equinox). Two hours later, the stars will have drifted 30˚ across the night sky. The term Right Ascension (RA) in effect represents the hours (angular distance eastwards) from the reference point that a particular celestial object will be. As an example, the celestial coordinates of the star BETELGEUSE in the constellation Orion are:Right Ascension = 5 hours 52 minutes : Declination = +7 degrees 24 minutes. There are many different ways that star maps can be presented. A map may show the stars for a month or over a year. The star map shown below shows the band of stars seen between about dec +30˚ and -30˚. BETELGEUSE Right Ascension = 5 hours 52 minutes : Declination = +7 degrees 24 minutes +30˚ 0˚ -30˚ 24 22 20 18 16 14 12 10 8 RIGHT ASCENSION (HOURS) More detail on star maps shows the constellations like Orion and usually gives information about the locations of other celestial objects found in the region eg star clusters like (M45) and nebulae (M42) 6 4 2 0