Our Place in the Cosmos

1. Our Place in the Cosmos Lecture 5 Phases of the Moon and Eclipses

2. Clarification • Sun rises and sets due east and due west respectively on the equinoxes • At our latitude, sun rises and sets further south in the winter, further north in the summer • Direction of sunrise changes by about 80 degrees through the year: 40 degrees south of due east at midwinter, 40 degrees north of due east at midsummer

3. Orbit of the Moon • The Moon orbits the Earth in a counterclockwise direction as viewed from Earth’s North Pole taking one sidereal month to complete an orbit • The Moon also rotates once on its axis each orbit so that the same face of the Moon is always seen from Earth • This is known as synchronous rotation and is a consequence of tidal effects

5. Phases of the Moon • The Moon is illuminated by reflected Sunlight, and the changing phases of the Moon are due what fraction of the Moon’s illuminated surface can be seen from Earth • A new Moon lies between us and the Sun - its unilluminated side is towards us and crosses the meridian at midday • A full Moon lies in the opposite direction to the Sun - its illuminated side faces us and crosses the meridian midnight

6. Sun

7. Phases of the Moon • Between new and full, the Moon is waxing (“growing in size and brilliance”) • By first quarter (a quarter of its orbit is completed) half of the visible Moon is illuminated and crosses the meridian near sunset - an evening moon is thus waxing • After full, the Moon is waning • By third quarter the other half of the Moon is illuminated and crosses the meridian around sunrise - a morning Moon is thus waning

8. Period of the Moon • The Moon takes 27.32 days to complete one orbit about the Earth - its sidereal period • However, Earth has completed about 1/12 of its orbit about the Sun in this time and so the phase of the Moon is not the same after one sidereal period • It takes 29.53 days between one full Moon and the next - its synodic period

9. Sidereal versus synodic period Animation

10. Eclipses • Solar eclipse - Earth passes through Moon’s shadow • Lunar eclipse - Moon passes through Earth’s shadow • A shadow has two parts • Umbra total shadow • Penumbra partial shadow • Type of eclipse depends where in the shadow the Earth or Moon lies

11. Solar Eclipse

12. Types of Solar Eclipse • Total no part of the Sun is visible • Partial only side of the Sun is visible • Annular central part of the Sun is blocked but outer ring is visible • We can get both total and annular eclipses since: • Sun and Moon have very similar angular size • Moon’s orbit is not perfectly circular • When Moon is further away it appears smaller and blocks only central part of Sun  annular eclipse

13. 2001 total solar eclipse 1999 annular solar eclipse

14. Partial v Total • Moon’s penumbra covers large area on Earth, so partial eclipses are quite common • Umbra is at most 269 km wide and so a total eclipse is a rare event • NB a total eclipse at one location may appear as a partial eclipse at another • A total solar eclipse lasts for a maximum of 7.5 minutes, usually significantly shorter

16. Lunar Eclipse • Because Earth’s shadow is much larger than Moon’s, lunar eclipses are both more common and longer-lived (up to 1h 40m) than solar • No annular lunar eclipses, only total or partial • Even during a total eclipse, Moon glows with a reddish appearance due to refraction of Sunlight by Earth’s atmosphere • Moon appears red as blue light is scattered by dust - same reason Sunset looks red

18. When do Eclipses Occur? • If Sun, Earth and Moon lay in exactly the same plane: • Moon would pass exactly between Sun and Earth every New Moon  solar eclipse • Earth would pass exactly between Sun and Moon every Full Moon  lunar eclipse • In fact Moon’s orbital plane is inclined by 5.2° relative to Earth’s orbit around Sun and so Sun-Earth-Moon are rarely co-linear

19. Eclipses are only possible when line of nodes points close to the Sun (eclipse season) Line of nodes: intersection of two orbital planes

20. Eclipse Season • Eclipse Season is not exactly every 6 months due to precession of the Moon’s orbit • Its orientation rotates once every 18.6 years opposite in direction to its orbit • Eclipse seasons occur about every 5 months and 20 days

21. Summary • Moon rotates as it orbits Earth such that we always see the same face (tidal locking) • Moon’s phases due to how much of illuminated surface is visible from Earth • Solar eclipse when Moon casts a shadow on the Earth • Lunar eclipse when Earth casts a shadow on the Moon • Eclipse season only twice per year due to inclination of Moon’s orbit to the ecliptic

22. Homework • Identify a work of art, literature or music that is influenced by the Moon or stars • In what ways is the depiction scientifically accurate? • In what ways is “artistic licence” taken? • Start thinking about topics for your talk and essay later this term (they can be the same)