Watching the Moon

Activity:

Lunar Cycles

Objectives:

In this Activity, we will investigate

(a) phases of the Moon,(b) the lunar sidereal & synodic periods, and(c) lunar and(d) solar eclipses.

Phases of the Moon

•The Moon shines by

reflected light•Like the planets, the Moon also follows a path close to the ecliptic across the night sky.

Instead of orbiting the Sun directly, it follows an almost circular path (e = 0.055) around the Earth, tilted at only 5o relative to the ecliptic.

As we watch the Moonin the sky from day today, we see it go through lunar phases - first quarter, full moon, third quarter and newmoon (or “no moon”)

Some people assume that the phases of the Moon are due to theshadow of the Earth falling partly on the Moon.

However once therelative positions of the Earth, Sun and Moon during the phasesare taken into account,

it is clear that the phasesof the Moon are due toour viewing varyingproportions of sunlight& shadow from the Sunduring each month.

The Moon orbits the Earth with a period of 27.3 days,always with thesame face turned towards us.

Rotation of the Moon

• The Moon orbits the Earth in a counter-clockwise direction if looking down on the Earth.

• The Moon itself is tidally locked with the Earth, which means that the Moon rotates on its own axis at the same rate as it rotates around the Earth thus showing only one face towards us at all times. This direction is also counter-clockwise.

The What and Why of the Dark Side of the Moon

https://planetariumweb.madison.k12.wi.us/files/planetarium/mooncal/coinrotationani2.gif

The period of 27.3 days, called the sidereal period - is relative to the “fixed stars”.

Earth

This means thatthe Moon goesfrom one particularposition in the night sky relative to the background stars,through its monthly cycle, till it gets back to that particular position again.

But the lunar cycle we usually notice, if we look at the night sky, is not related to the moon’s position relative to the fixed stars.

What is it?

The most noticeable lunar cycle is the time from one full moon to the next - or the time from one “moonless” night (“new moon”) to the next.

The length of this cycle is called the “synodic period”, and takes longer: 29.5 days, compared to 27.3 days for the sidereal period.

The synodic period is the interval between identical lunar phases.

This means that the Moon has to travel a bit further to get back into the same orientation relative to the Sun & Earth- otherwise it wouldn’t be showing the same phase.

The synodic period is longer than the sidereal period, because, while the Moon is going throughits monthly cycle, the Earth has moved approx. 1/12th of the way around its orbit.

Neither the sidereal or synodic period corresponds exactly to our calendar month.

The synodic period, 29.5 days, is close to the length of an average month, but the reason why some months have 31 days while others have 30 (and February is left with only 28, most years) has more to do with ancient Roman politics than it does with astronomy.

Phases of the Moon

New Moon – (no Moon) Moon positioned between the earth and the

sun.

Full Moon – Moon on opposite side of the earth

First Quarter and Third Quarter – (called half moons) Moon at 90

degree angle with respect to earth and sun.

…the phases in between..Crescent and Gibbous

Crescent – less than half illuminated

Gibbous – more than half illuminated

Waxing ( growing), Waning (shrinking)

Review Motion of the Moon

Moon takes about 29.5 days to go through whole cycle of phases – synodic month

Phases are due to different amounts of sunlit portion being visible from Earth

Time to make full 360° around Earth, sidereal month, is about 2 days shorter

Lunar eclipses

When the Moon moves into the Earth’s shadow,a lunar eclipse occurs. Can only occur during Full Moon.

The Moon’s phases are not due to shadowing by the Earth, but lunar eclipses are.

While in the Earth’s shadow, the Moon is seen fromEarth as darkened and somewhat reddened.

The reddening effect comes about for the same reasonthat sunsets are red:

The Earth’s atmosphere scatters muchof the Sun’s light, but the red end of the spectrum

As the Sun’s light takes its long paththrough the atmosphere at the “edges” of the Earth:

The Earth’s atmosphere scatters muchof the Sun’s light, but the red end of the spectrum

Most of the spectrum is scattered away,leaving only the red end of the spectrumto be transmitted through the atmosphere:

… and the reddish light faintly illuminates the Moonwhile it is within the Earth’s shadow.

The Earth’s atmosphere scatters much of the Sun’s light, but some of the red end of the spectrum is transmitted

What would you expect to see if you were on the Moonduring a lunar eclipse?

If you were on the near side of the Moon, looking at the Earth while a lunar eclipse was taking place,the Earth would appear dark, surrounded by a ringof reddish-tinged atmosphere.

Solar eclipses

Although the Sun is much larger than the Moon,it is also much further away. By a convenient coincidence, the angular diameter of the Sun & Moon are both almost the same - approx. 0.5o.

Solar Eclipses only occur with a new Moon

Solar eclipses

Because of this coincidence, we are occasionally treatedto one of nature’s most spectacular events- a total solar eclipse.

When the Moon moves directly (or almost directly)between the Sun & the Earth, a solar eclipse occurs.

The Moon’s shadow traces a path over the Earth:observers where the central dark part of the shadow(the umbra) passes see a total eclipse.

Observers in the grey “rim” of the shadow will see the Sun partly but not completely obscured - a partial solar eclipse.

Here are three images of a partial solar eclipse takenfrom Kangaroo Island, South Australia on 2 Sept, 1997

© Dan Staiger

(The red circle is highlighting the location of a sunspot)

Solar eclipses might be expected to happen somewhere on Earth every month, but the 5o tilt of the Moon’s orbital plane limits the chances of the Sun, Moon and Earth being in alignment.

Total solar eclipses give us a chance to observe the faint but beautiful outer layers of the Sun - the chromosphere and the corona - normally “drowned out” by the bright solar photosphere.

© Bill Ronald

Summary Motion of the Moon

Eclipses occur when Earth, Moon, and Sun form a straight line

Summary- Motion of the Moon

Lunar eclipse:

• Earth is between Moon and Sun

• partial when only part of Moon is in shadow

• total when it all is

1.6 Motion of the Moon

Solar eclipse: Moon is between Earth and Sun

• partial when only part of Sun is blocked

• total when it all is

The Changing Size of the Moon

•Apogee is the furthest point from the earth.

•Perigee is the closest point to the earth and it is in this stage that the moon appears larger. Looking at the moon in the sky without anything to compare it to, you wouldn't notice any size difference. But the difference in size can in fact be quite significant.

•If you were to photograph a full moon at apogee and perigee (using the same lens), here's how the two sizes would compare: