AS101 LESSON 2

Cycle of Sun (daily)

Sun appears to move east to west because Earth rotates

Cycle of Sun (annual)

Sun appears to move eastward relative to stars because Earth revolves around Sun

Cause of seasons

Earth’s 23.5° axial tilt, not distance from Sun

Summer conditions

More direct sunlight + longer daylight hours

Winter conditions

Less direct sunlight + shorter daylight hours

Summer solstice

Longest day of year in Northern Hemisphere

Winter solstice

Shortest day of year in Northern Hemisphere

Equinox

Day and night nearly equal

Moon phase cycle length

29.5 days

Cause of Moon phases

We see different portions of Moon’s sunlit half as it orbits Earth

How much of Moon is always illuminated

Half

New Moon

Moon between Earth and Sun

Full Moon

Moon opposite Sun with Earth between

First Quarter

Half lit and waxing

Third Quarter

Half lit and waning

Waxing vs waning

Waxing increasing illumination; waning decreasing illumination

Crescent vs gibbous

Crescent less than half lit; gibbous more than half lit

Lunar eclipse

Earth’s shadow falls on Moon during full moon

Types of lunar eclipses

Penumbral, partial, total

Penumbral lunar eclipse

Moon passes through penumbra only

Partial lunar eclipse

Part of Moon enters umbra

Total lunar eclipse

Entire Moon enters umbra

Solar eclipse

Moon’s shadow falls on Earth during new moon

Types of solar eclipses

Partial, total, annular

Partial solar eclipse

Only part of Sun covered

Total solar eclipse

Sun fully covered for observers in umbra

Annular solar eclipse

Ring of Sun visible because Moon appears smaller

Why eclipses do not happen monthly

Moon’s orbit tilted about 5°

Solar coordinates

Altitude and azimuth used to locate Sun in sky

Altitude

Angle above horizon

Azimuth

Compass direction along horizon

General motion of Sun, Moon, planets relative to stars

Usually eastward

Retrograde motion

Temporary apparent westward motion of a planet

Cause of retrograde motion

Different orbital speeds create apparent reversal

Stellar parallax

Apparent shift of nearby star against distant stars

Cause of stellar parallax

Earth viewed from different positions in orbit

Importance of stellar parallax

Measures nearby star distances and supports Earth’s orbit around Sun