The Moon Topics: • Why do we only see one face of the Moon? • Why do we see phases of the Moon? • What causes eclipses?
Phases of Moon • Half of the Moon is illuminated by the Sun and half is dark. • We see a changing combination of the bright and dark faces as the Moon orbits Earth. How to Simulate Lunar Phases
Phases of the Moon Phases of the Moon
Phases of the Moon: 29.5-day cycle new crescent first quarter gibbous full gibbous last quarter crescent } • waxing • Moon visible in afternoon/evening • Gets “fuller” and rises later each day } • waning • Moon visible in late night/morning • Gets “less” and sets later each day
Moon Rise/Set by Phase Time the Moon Rises and Sets for Different Phases
Thought Question It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? • First quarter • Waxing gibbous • Third quarter • Half moon
Thought Question It’s 9 A.M.You look up in the sky and see a moon with half its face bright and half dark. What phase is it? • First quarter • Waxing gibbous • Third quarter • Half moon
What causes eclipses? - Shadows • The Sun is the source of visible light in the solar system • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse.
When can eclipses occur? • Lunar eclipses can occur only at full moon. • Lunar eclipses can be penumbral, partial, or total.
Solar Eclipse Evolution of a Total Solar Eclipse
When can eclipses occur? • Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, or annular.
Why don’t we have an eclipse at every new and full moon? • The Moon’s orbit is tilted 5° to ecliptic plane. • So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.
Eclipses - Shadows • Lunar Eclipse • Moon passing through Earth’s shadow • Full moon • Three types: • Total - through umbra • Partial - partially through umbra • Penumbral - through penumbra • Solar Eclipse • Moon’s shadow crosses the Earth • New moon • Three types: • Total - umbra • Partial - penumbra (outside misses earth) • Annular - Moon farther from Earth, umbra doesn’t reach Earth
Planets Known in Ancient Times • Mercury • difficult to see; always close to Sun in sky • Venus • very bright when visible; morning or evening “star” • Mars • noticeably red • Jupiter • very bright • Saturn • moderately bright
Sun Norse Sol Sunday Mars Norse Tyr Tuesday Venus Norse Frigga Friday Mercury Germanic Woden Wednesday Jupiter Norse Thor Thursday Moon Germanic Mani Monday Saturn Roman Saturday Type in “week” in Wikipedia…and also “days of the week”. Why are there 7 days in a week? The ancient Babylonians are known to have observed a seven-day week; each day dedicated to a different deity. The significance of seven comes from Babylonian astronomy. There are the seven heavenly bodies or luminaries normally visible to the naked eye (the Sun, Moon, and 5 visible planets), and they associated each with a deity. The word planet means “wanderer” in Greek.
What was once so mysterious about the movement of planets in our sky? • Planets usually move slightly eastwardfrom night to night relative to the stars. • But, sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion.
We see apparent retrograde motion when we pass by a planet in its orbit. Mars Retrograde Motion
Explaining Apparent Retrograde Motion • Easy for us to explain: this occurs when we “lap” another planet (or when Mercury or Venus laps us). • But it is very difficult to explain if you think that Earth is the center of the universe! • If the Earth is moving, why don’t we see parallax in the positions of the stars?