Our Place Our Place in in the the Cosmos Cosmos further south - - PDF document

Our Our Place Place in in the the Cosmos Cosmos

Lecture 5 Phases of the Moon and Eclipses

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

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

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

Sun

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

Period of the Moon

• The Moon takes 27.32 days to complete
• ne 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

Sidereal versus synodic period

Animation

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

Solar Eclipse 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

2001 total solar eclipse 1999 annular solar eclipse

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

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

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

Line of nodes: intersection of two orbital planes

Eclipses are only possible when line of nodes points close to the Sun (eclipse season)

Eclipse Season

• Eclipse Season is not exactly every 6

months due to precession of the Moon’s

• rbit
• Its orientation rotates once every 18.6

years opposite in direction to its orbit

• Eclipse seasons occur about every 5

months and 20 days

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

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)