the sky lrg

Grunt Productions 2000

The Sky

A brief by Lance GrindleyA brief by Lance Grindley


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.


Horizon and Zenith


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.– 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.


Magnetic North Pole


Magnetic North


Correction from Magnetic North to True North



• The altitude and azimuth of a star change during the course of night as the star rises and sets.

• Angles are measured using degrees, minutes of arc, and seconds of arc.


Measuring Angles in the Sky


Measuring Angles

• Angles in astronomy are measured in degrees, arc minutes and arc seconds.– 1 degree = 60 arc minutes– 1 arc minute = 60 arc seconds– 1 degree = 3600 arc seconds– 1 arc second is the size of a 50p as seen from 5

km (3 miles) or a penny as seen from 2.2 miles.


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.


Angular Sizes and Distances


The Celestial Sphere


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.


Longitude and Latitude


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.


Rotation of the Earth


The stars move from east to west because the earth rotates from west to east.


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.


Daily (Diurnal) Motion of the Stars


Circumpolar Stars


Diurnal Paths of Stars


Diurnal Paths of Stars at Intermediate Latitude


Celestial Poles


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.


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.


Diurnal Paths of Stars


Celestial Sphere Movie

http://brahms.phy.vanderbilt.edu/~rknop/astromovies/


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, arc minutes, and arc seconds. 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.



Vega

RA 18h35m

DEC +38o44’


Exercise

• Find Orion in the night sky. Is it on the equator, the ecliptic, both, or neither.

• How long is Orion above the horizon each day?

• Find Sagittarius. Is it north or south of the equator?

• How long is Sagittarius above the horizon each day?

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