Celestial Coordinates

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One of the very first things you have to be able to do if you are going to begin a hobby in astronomy is learn to read a star map. My astronomy class spends at least the first two class periods learning how to do just that. It is as important as knowing how to read any other map. Imagine trying to travel to a new place in the United States without knowing how to read a map. Try to find a constellation or a deep sky object without knowing how to read a sky map. This is one of the foundations. I have explained a few of the more important terms you will need to know. These terms are used in every astronomy book and magazine you will pick up. You will need to become familar with these terms.

Imagine that all of the stars and planets and moon and sun lie on the surface of an imaginary sphere surrounding the Earth. At one time ancient people believed this to be the case. Astronomers now know that the heavens are not really a sphere, but it is convenient to imagine the this is so because that is the way it looks to us on the earth. This sphere is called the Celestial Sphere. The poles of the Celestial Sphere lie directly above the poles of the Earth., above the Earth's North Pole is the north pole of the Celestial Equator and above the Earth's South Pole is the south pole of the celestial equator. We in the northern hemisphere have a star marking the north celestial pole called Polaris, the North Star. The celestial equator lies directly above the Earth's equator.

Now get a globe and a star map to look at as you read the rest of this page. You really must know this stuff, so put on your thinking caps and get started. This is quoted from The Practical Astronomer by Colin A Ronan, Bonanza Books, New York, page 11.

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"Charting stars on the celestial sphere is carried out in a similar way as mapping places on the Earth's surface. On the Earth, we fit a 'grid' of lines from which we measure places. We draw in the Earth's north and south poles and the equator, the line cutting the Earth in half between the poles and separating the northern and southern hemispheres. Then we draw a line from the north pole to the south pole though Greenwich, just south of London. This is the 'Greenwich meridian', and where it crosses the equator is the starting point of our measurements. To find the position of any place, say New York, we draw a meridian through the place and then measure the angle westwards along the Greenwich meridian and the median through New York. It is 74o . This gives us the longitude. We still have to find the distance of the place north or south of the equator. New York is north of the equator, and the angle along its meridian, measured upwards from the equator to pole, gives us the latitude. New York's latitude is almost 41o north. So we can give the position of New York by quoting two coordinates: Latitude 41o north, Longitude 74o west. That gives us New York's position without any possibility of confusion.

"To chart the positions of stars, a similar system is used. On the celestial sphere we draw the north and south celestial poles, and the celestial equator, half way between them. But what do we use instead of the meridian of Greenwich as our fixed point on the celestial equator? The most convenient thing to do is to use the Sun's apparent path as our guide. During a year the Sun appears to move across the background of stars: its path is known as the ecliptic (because it is connected with determining eclipses). In the northern hemisphere spring, this path cuts the celestial equator as the Sun moves north of it. During the northern summer the Sun stays north of the equator (which is why it is summer), then it dips in the autumn crossing through the celestial equator again. When the Sun is on the celestial equator, night and day are equal in length, and each crossing point is often referred to as an equinox.

The crossing point of ecliptic and celestial equator which the Sun reaches in the northern spring is the vernal equinox (also known as The First Point of Aries) and denoted by the symbol U. Using this as our 'Greenwich' equivalent we can measure the position of any star. Astronomers use the term right ascension (RA) for the celestial equivalent of longitude and express their measurements, which are made only in an eastwards direction, in hours, minutes, and seconds of time instead of degrees. This is because the celestial sphere appears to rotate with time (we imagine the Earth as stationary). The whole way round the celestial equator (360o) is taken as 24 hours, so star X will have ascension of 06 hours 00 minutes 00 seconds. The astronomical equivalent of terrestrial latitude is called declination, and this is measured in degrees and fractions of a degree. Thus the declination of star X is +45o00'00". A minus sign before the declination would mean the star lies south of the celestial equator. so the position of X in the sky can be precisely specified, just as easily as a place on the Earth. "

The following is quoted from American Nature Guides: Astronomy by Ian Ridpath, Galley Books New York, page 10-11.

"Distances on the celestial sphere are measured as angles. Angles smaller than a degree are measured in arc minutes, one arc minute being one-sixtieth of a degree. For example, the diameters of the Moon and Sun are each about half a degree, or 30 arc minutes. The unaided human eye can distinguish objects as close as 2 or 3 minutes apart.

The smallest angles of all, such as the separation between two close stars or the apparent diameter of a planet, are measured in arc seconds, often abbreviated arcsec. An arc second is one-sixtieth of an arc minute, and corresponds roughly to the width of a small coin observed at a distance of 4 km. Jupiter has the largest apparent diameter of any planet, about 50 arcsec when at its closest to us."

The following is quoted from Skyguide; A Filed Guide to the Heavens by Mark R. Chartrand III, Golden Press, New York, page 10.

"Your celestial sphere has a spot unique for you: the point directly overhead, the zenith. If you move, your zenith moves. No other location on Earth has the same zenith as yours. The point opposite the zenith on the celestial sphere is the nadir.

The location of a celestial object with respect to the observer's location is described by the two angles called altitude and azimuth.

Altitude is the angle between the horizon and the object. This angle is measured from the horizon and perpendicular to it. Altitude is not height above the ground. It is an angle centered on you; or, an arc along the celestial sphere. Altitude ranges from 0o at the horizon to +90o at the zenith. Objects below the horizon have negative altitudes. Objects at the same altitude are said to be on a parallel of altitude.

Azimuth is the compass direction toward an object. More precisely, it is the angle measured around the horizon, beginning at north (0o) through east (90o) and so on until you reach a line drawn perpendicular from the object to the horizon. Thus east is azimuth 90o, south 108o, west 270o, and north 360o--the same as 0o."

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Now take a deep breath as we review the important vocabulary for this unit.

altitude a. distance above the surface of a planet, used in describing an atmosphere or a spacecraft orbit. b. Angle of elevation above the horizon, for the line of sight to a celestial object.

Azimuth. Angle measured clockwise around the horizon, from 0* towards the north, through 90* to the east, 180* to the south, 270* to the west, and to 360* due north.

Celestial Equator. Great circle formed on the celestial sphere by the extension out of the Earthís equatorial plane.

Celestial Sphere. Apparent spherical surface, centered on the Earth to which the stars seem to be fixed.

Declination. Angular distance of an object north or south of the celestial equator, measured in degrees. Thus the north celestial pole has a declination of +90 degrees.

Ecliptic. Path that the Sun appears to follow, against the stars on the celestial sphere, during the course of a year.

Ecliptic plane. Plane defined by the Earthís orbit around the Sun.

Equinox. Two days each year when the Sun is above and below the horizon for equal lengths of time.

Meridian. Great circle, on the celestial sphere or the Earth, that passes through both north and south poles and an observerís zenith or location.

nadirThe point opposite the zenith on the celestialsphere.

Solstice. Extreme positions reached by the Sun, north and south of the celestial equator. When the Sun is at these positions, the shadows it casts on the Earthís surface are the longest or shortest (depending on whether you are making this observation in the northern or southern hemisphere).

Summer solstice. extreme northern position of the Sun on the celestial sphere, north of the celestial equator (summer refers to the northern hemisphere).

Winter solstice. Extreme southern position of the Sun on the celestial sphere, south of the celestial equator (winter refers to the northern hemisphere).

Zenith. Point on the sky directly overhead.

Good links for more on this subject.

Understanding Celestial Coordinates from Sky and Telescope--Good article.
Latitude Is Everything --Another good article from Sky and Telescope's web site.

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