Download Astronomy 110 Lecture 2.

Astronomy 110
Lecture 2.
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We each define a local horizon plane that is tangent to the Earth. It
defines up and down and divides the sky into observable (above) and
unobservable (below) halves
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Azimuth
Zenith and Azimuth Angles Determine a Local Coordinate System
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The Earth’s Spin Defines Natural References For Location
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We project the naturally defined markers on the Earth (poles and
the Equator) onto the sky to begin to establish a coordinate
system on the celestial sphere.
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In the Northern hemisphere the star Polaris is currently
(accidentally) very close to the North Celestial Pole.
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There is no such accidental alignment in the Southern hemisphere
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Why do stars rise and set?
The Earth rotates east to west, so
stars appear to circle from west
to east.
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Coordinates on the Earth
• Latitude: position north or south of equator
• Longitude: position east or west of prime meridian
(runs through Greenwich, England)
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The Night Sky Rotates as if stars were stuck on a rigid rotating
sphere – The Celestial Sphere
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What are constellations?
A constellation is a
region of the sky.
88 constellations
fill the entire sky.
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The Celestial Sphere
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Angle measurements:
• Full circle = 360º
• 1º = 60′ (arcminutes)
• 1′ = 60″ (arcseconds)
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How Long is a Day?
• Solar day: The time it takes the Sun to return to the same
position in the sky = 24 hours
• Sidereal day: The time it takes a star to return to the same
position in the sky = Earth’s rotation period = 23hr, 56min
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The Analemma
The Solar day is not constant because the Earth’s motion with
respect to the Sun is not constant
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Why don’t we see the same
constellations throughout the year?
1. The Night Sky Varies With Latitude.
2. The Night Sky Varies With the Time of Year.
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The Altitude of the Celestial Pole is Your
Latitude
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The sky varies as Earth orbits the Sun
• As the Earth orbits the Sun, the Sun appears to move eastward
along the ecliptic.
• At midnight, the stars on our meridian are opposite the Sun the
in the sky.
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What we see of the night sky is
determined by the intersection
of our local horizon with the
celestial sphere.
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Some stars never cross the observer’s horizon and thus never appear to rise or set
so they are always either above or below the horizon.
If they are above the horizon they are called circumpolar and are always visible
throughout the year.
If they are below the horizon – they cannot be seen from that latitude.
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Our view from Earth:
In general, for observers in the northern hemisphere:
• Some stars are near enough to the north celestial pole that they are circumpolar and
never set.
• Some stars are near enough to the south celestial pole that they can never be observed
from that latitude.
• All other stars as well as the Sun, Moon and planets rise in east and set in west.
Celestial Equator
Your horizon
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How Can We find Our Latitude?
1. Paths of Stars as they Set
2. Altitude of the Pole
3. Region of the Sky that is Circumpolar
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