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Date ___________________________________

Before doing anything else, find where our college is located. Our Earth based latitude and longitude are respectively: (roughly) +45 degrees North and 093 degrees West. Likely all you can find is St. Paul or maybe just Minnesota.

Horizon Coordinate System

1. Locate and define the zenith on the celestial sphere.

2. Locate the nadir on the celestial sphere.

3. Rotate the celestial sphere, notice that the stars on the zenith and nadir change as the celestial sphere rotates.

4. How long does it take for the celestial sphere to turn once around relative to the stars? __________ This is a sidereal day. It is not quite 24 hours. What is it exactly? ___________ What causes it to be different from 24 hours? Explain in a sentence or two or a labeled sketch.

What kind of day does your watch or a good clock keep anyway? ______________

5. The coordinate altitude in the astronomical coordinate system called the Horizon system is measured from the horizon up or down. The altitude of the horizon is zero. When the altitude is above the horizon it is denoted as a positive angle. When the altitude is below the horizon it is denoted as a negative angle. What is the altitude of the zenith in degrees? _________ What is the altitude of the nadir in degrees? _________ What is the altitude half way up in the sky in degrees? What is the altitude of the north pole from Takoma Park? _________What has the altitude of the north pole got to do with the latitude of any place in the northern hemisphere? __________ What has the altitude of the south pole got to do with the latitude of any place in the southern hemisphere? __________ What is the altitude of the north pole on the equator of the earth? ___________ What is the altitude of the south pole on the equator? _____________

8. What is the altitude of the sun at sun rise? ___________What is the altitude of the sun at sun set? ________

9. The great arc which runs from the north point on the horizon to the zenith and from the zenith to the south point on the horizon is called the Meridian. How many degrees are there in this meridian? __________ When an object in the sky---like the sun, moon, or stars---crosses the meridian it is said to transit the meridian. Objects which are near enough to the poles never raise or set, they are said to be circumpolar. Since we live in the northern hemisphere with a latitude of about 45 degrees, then celestial objects within about 45 degrees of the north pole never raise or set they are always above our horizon. These circumpolar objects cross the meridian twice. Once when they are lowest in the sky and once when they are highest in the sky. Notice this on your celestial sphere with the pointer stars in the big dipper, ursa major, Duhbe and Merack. Likewise their are circumpolar celestial objects that are 39 from south pole which never raise or set they are always below our horizon, so we can never see these circumpolar objects. Centauri, the brightest star in the constellation of Centarus and the closest star to the earth other than the sun---about 4 light years away (the distance that light can travel in 4 years)---is such a circumpolar star near enough to the south pole as to be always below our horizon and therefore always invisible to us in the Twin Citiesmetro area. Which direction would you have to go to be able to see Centauri?

Most celestial objects are not circumpolar from the Twin Cities metro area. Where on the earth are all of the stars circumpolar? _________ Notice in the Twin Cities metro area the sun which moves along the ecliptic, the moon, and all the planets which move near the ecliptic are never circumpolar. Since the sun moves along the ecliptic, because the earth goes around the sun as we currently understand, where on the earth---in what region of the earth---is the sun sometime during the year circumpolar.________________

10. The sun is never over head in the Twin Cities metro area. Rotate the celestial sphere around while it is rectified for our latitude and notice that the ecliptic is never overhead. What day does the sun transit the meridian at its lowest? ___________ What is the altitude of the sun when it transits at is lowest? ___________ No wonder this is such a depressing day. On this day when it transits at its lowest what direction does it raise from? On this day when it transits at its lowest what direction does it set from? ____________ What day does the sun transit the meridian at the highest? ____________ What is the altitude of the sun when it transits at it highest? _________ It is little wonder that this day is seldom depressing.

On this day when it transits at its highest what direction does it raise from? ____________On this day when it transits at its highest what direction does it set from? ____________ On what two days does the sun raise exactly from the east and set exactly from the west? _________ By the way that means on all of the other day that the sun is either north or south of east and west when it raises or sets. Are you surprised? ______ Most people who have never been nature priests or astronomers are surprised. This coordinate system is based upon the physical fact that we live on the earth and gravity pulls us to the earth establishing and up and a down locally. How high up in angle is called altitude and how far over along the horizon is called azimuth!

Some Terms You Should Know: the Celestial Sphere
These are some terms you will come across in your lab class dealing with the starglobe and locating celestial objects.

Altitude
The height, in degrees, of a star's position with respect to the horizon along a line running from the horizon to the zenith. The coordinates of a star can be given in Azimuth and Altitude. These numbers will change as the star rises and sets.

Autumnal Equinoctal (or Fall Equinoctal)
The celestial meridian (an imaginary line) that is overhead at noon on the first day of autumn. Stars on the Autumnal Equinoctal have a Right Ascension of 12 hours.

Azimuth
The angle, in degrees, of a star's position around the horizon clockwise from due north. A star due east has an azimuth of 90 degrees. The coordinates of a star can be given in Azimuth and Altitude. These numbers will change as the star rises and sets. Go here for more information.

Celestial Sphere
An imaginary, transparent sphere with the Earth at it's center; all of the stars are pictured as being on this sphere. This is what you see on the star globe.

Celestial Pole
The axis around which the celestial sphere rotates. As the Earth rotates, the stars appear to circle the celestial pole.

Celestial Equator
The imaginary line on the celestial sphere directly above the Earth's equator halfway between the north and south clestial poles.

Declination
The angle, in degrees, of a star's position above or below the celestial equator. (Declination is negative for a star below the celestial equator.) A star on the celestial equator has a declination of 0 degrees; a star at the north celestial pole has a declination of 90 degrees. The coordinates of a star can be given in Right Ascension and Declination. These numbers are with respect to the celestial sphere and do not change. Go here for more information.

Ecliptic
The plane of the Earth's orbit around the sun. On a star map or star globe, the apparent path of the sun across the celestial sphere.

Equator
The imaginary line around the center of the Earth halfway between the north and south poles.

Latitude
The angle, in degrees, of a position on Earth above the equator. (Latitude is negative for a place below the equator.) Points on the Earth can be located by Latitude and Longitude.

Longitude
The angle, in degrees, of a position on Earth around the equator from the Greenwich Meridian. Points on the Earth can be located by Latitude and Longitude.

Meridian
An imaginary line in the celestial sphere that goes from the north celestial pole through the zenith to the south celestial pole. The north-south line directly overhead.

Right Ascension
The angle, in hours, of a star's position around the celestial equator with respect to the Vernal Equinoctal (an imaginary line on the celestial sphere). The coordinates of a star can be given in Right Ascension and Declination. These numbers are with respect to the celestial sphere and do not change.

Sidereal Day and Sidereal Time
The day and time measured with respect to the stars.

Solar Day and Solar Time
The day and time measured with repect to the sun.

Universal Time
The solar time at a longitude of 0 degrees. Often called Greenwich Mean Time, because the British Royal Observatory in Greenwich sits on 0 degrees longitude.

Vernal Equinoctal (or Spring Equinoctal)
The celestial meridian (an imaginary line) that is overhead at noon on the first day of spring. Stars on the Vernal Equinoctal have a Right Ascension of 0 hours.

Zenith
The point in the sky directly above the observer.