1. 12/29/2018
The Sky

2. Measuring Angles
Angles in astronomy are measured in degrees, arcminutes and arcseconds.
1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
1 degree = 3600 arcseconds
1 arcsecond is the size of a U.S. quarter as seen from 5 km (3 miles) or a penny as seen from 2.2 miles.
12/29/2018
The Sky

3. Measuring Angles in the Sky
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4. Measuring Angles The Sun and Moon appear to be about ½ degree in size.
Your finger held at arms length is about one degree across.
Your fist at arms length is about 10 degrees.
Your outstretched hand at arms length is about 20 degrees across.
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The Sky

5. Angular Sizes and Distances
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6. Horizon and Zenith
It is sometimes useful to think of the sky as a great dome over our heads.
The horizon is where the dome meets the Earth.
The zenith is the point directly overhead.
As the Earth turns, this dome turns over our heads. It appears as if the sky is a large hollow sphere centered on the Earth.
12/29/2018
The Sky

7. Horizon and Zenith
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8. Altitude and Azimuth
The height of a star above the horizon is called the altitude.
The direction to the star as measured from true north is called the azimuth.
True north is at an azimuth of 0 degrees.
East is 90 degrees
South is 180 degrees
West is 270 degrees
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9. Altitude and Azimuth
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10. In this previous picture, the Sun is in the south east. Its azimuth is
45 degrees
60 degrees
90 degrees
135 degrees
180 degrees
225 degrees
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11. Here in Virginia, a compass points true north.
False
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12. Altitude and Azimuth
Note: True north is not the same as magnetic north. The magnetic north pole is not located in the same place as the true north pole.
On maps, the legend will show you how to correct from magnetic north, as measured by a compass, to true north.
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13. Magnetic North Pole
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14. Magnetic North
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15. Correction from Magnetic North to True North
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16. Altitude and Azimuth
The altitude and azimuth of a star change during the course of night as the star rises and sets.
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17. Longitude and Latitude
Latitude: Your north-south position on Earth. The equator is defined to have a latitude of 0o. The north pole is at 90oN and the south pole at 90oS.
Longitude: Your east-west position on Earth. An arbitrary point, the Prime Meridian in Greenwich, England marks a longitude of 0o.
Leander McCormick Observatory is at:
Longitude 78o 31’ 19.8” W
Latitude 38o 01’ 58.2” N
Altitude 264 meters
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18. Longitude and Latitude
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19. 78.5o east, 38o north 78.5o east, 38o south 101.5o east, 38o south
Charlottesville is roughly at 78.5o west and 38o north. If you dug a hole straight through the Earth, where would you come out?
78.5o east, 38o north
78.5o east, 38o south
101.5o east, 38o south
101.5o west, 38o south
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20. Where is this? China India Southern Pacific Ocean Indian Ocean
Antartica
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21. The Celestial Sphere
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22. The Celestial Sphere
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23. The Celestial Sphere
North Celestial Pole: The point directly over the Earth’s true north pole.
The north star, Polaris, is near the North Celestial Pole, but not exactly at the pole. It is currently about 1 degree away from the pole.
South Celestial Pole: The point directly over the Earth’s true south pole.
Celestial Equator: The equator of the Earth projected onto the celestial sphere.
Meridian: A line from due north to due south that passes straight overhead.
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The Sky

24. North Celestial Pole
In the northern hemisphere, the altitude of the north celestial pole is equal to your latitude on Earth.
This is useful for navigation.
If you measure the altitude of the north celestial pole, you can determine your latitude on Earth.
In the southern hemisphere, it is difficult, but not impossible, to find the location of the south celestial pole.
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The Sky

25. The stars move from east to west because the earth rotates from west to east.
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26. Celestial Poles
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27. Celestial Sphere
To find due north, drop straight down from the North Celestial Pole to the horizon.
The celestial equator meets the horizon at due east and due west.
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28. Diurnal Paths of Stars
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29. The Motion of the Stars
Just like the Sun and Moon the stars rise and set due to the rotation of the Earth.
They rise in the east and set in the west because Earth rotates from west to east.
Stars near the celestial poles do not rise or set. Instead they circle the poles and are called circumpolar.
In the northern hemisphere, the stars circle the pole in a counterclockwise direction.
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30. Rotation of the Earth
1300 km/hr = 800 mph
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31. Diurnal Paths of Stars
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32. Diurnal Paths of Stars
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33. Celestial Coordinates
Just as Earth has lines of longitude and latitude, the celestial sphere has a system of celestial coordinates:
Declination (dec): The north-south position of a star on the celestial sphere. Declination is measured in degrees, arcminutes, and arcseconds. The celestial equator is defined to have a declination of 0o.
Right Ascension (RA): The east-west coordinates of an object on the celestial sphere. R.A. is measured in hours, minutes, and seconds of time. The vernal equinox is defined to be 0h.
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34. Celestial Coordinates
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35. Celestial Coordinates
Vega
RA 18h35m
DEC +38o44’
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36. 12/29/2018
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37. These stars are always above the horizon (circumpolar).
These stars are above the horizon for more than ½ of a day.
Stars on the equator are above the horizon for exactly ½ of a day (12 hours).
These stars are above the horizon for less than ½ of a day.
These stars are never above the horizon.
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38. 0 hours 6 hours 9 hours 12 hours 15 hours 18 hours 24 hours
As seen from Charlottesville, a star on the equator (DEC=0o)will spend how many hours above the horizon?
0 hours
6 hours
9 hours
12 hours
15 hours
18 hours
24 hours
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39. 0 hours 6 hours 9 hours 12 hours 15 hours 20 hours 24 hours
As seen from Charlottesville, a star at DEC=40o will spend how many hours above the horizon?
0 hours
6 hours
9 hours
12 hours
15 hours
20 hours
24 hours
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The Sky