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Lecture 5

AST 101 Lecture Notes - Lecture 5: Solar Time, Sidereal Time

Course Code
AST 101
Walter Freeman

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Section 2.2 – The Reason for the Seasons
The tilt of earths axis causes sunlight to fall differently on Earth different times of the
oThe axis relative to the sun changes over the year
oTwo hemispheres – opposite seasons
oSteeper angle – higher path the sun follows = more daylight hours in summer
Solstices and equinoxes
oJune (summer) solstice – Nhemi is tipped most directly to the sun (June 21)
oDecember (winter) solstice – Nhemi receives the least direct sunlight
oMarch (spring) equinox – Nhemi goes from being tipped away from the Sun to
towards the sun
oSeptember (fall) equinox – Nhemi first starts to be tipped away from the Sun
Equinox = equal night
The seasons
The earth’s axis is not lined up with its orbital axis
Tilted by 23.4 degrees
Axis of rotation changes only very slowly – axis is off on zodiac
Noon each day, the sun will rise high and low In the sky, not east and west relative to the stars
Sidereal day is shorter than solar day
Solar day – suns east west position stays fixed but stars move east
Sidereal day – stars position stays fixed but sun moves west
Solstice- time of year when earths axis is tilted toward the sun the most
Equinox – periods where the earths tilt is sideways to the sun
Day and night have equal length
Tropics – region on earth where the sun alternates between the northern sky and the southern
Tropic of cancer – northern boundary
oJune solstice, zenith hits this
Tropic of Capricorn – southern boundary
oDecember solstice, zenith hits this
Arctic (north) Antarctic (south) – region where sun never rises or sets during certain parts of the
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