Module 5: Watching the Moon

Activity 1:

Lunar Cycles

Summary

In this Activity, we will investigate

(a) phases of the Moon,

(b) the lunar sidereal & synodic periods,

(c) lunar eclipses, and

(d) solar eclipses.

(a) Phases of the Moon

Like the planets, the Moon also follows a path close to the ecliptic acrossthe night sky.

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

Watching the Moon from day to day, we see it go through lunar phases - first quarter, full moon, third quarter and new moon (or “no moon”)

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

However, once therelative positions of the Earth, Sun and Moon are taken into account, it is clear that the phases of the Moon are due to the varying amounts of sunlight & shadow on the Moon as seen from the Earth each month.

Phases of the Moon

A

BC

D

E

FG

H

Sun’s

rays New moon Full moon

Third quarter

First quarter

A B C D E F G H

Phases as seen from Earth

On the Internet, you can use John Walker’s “Earth and Moon viewer” site,http://www.fourmilab.ch/earthview/vplanet.html

to see how the Moon would look from both the Earth and from the Sun right now.

It also shows how the Earth would look from the nearside of the Moon.

(Note that the far side of the Moon is not necessarily the dark side of the Moon.)

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

(b) Lunar Sidereal & Synodic Periods

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

This is the time it takes for the Moon to go from one particularposition in the night sky relative to the background stars,through its monthly cycle, and back to the same position relative to the stars again.

One sidereal period

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 background 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 it takes longer: 29.5 days compared to 27.3 days for the sidereal period.

The synodic period is the interval between identical lunar phases.

Phases of the moon.

This means that the Moon has to travel a bit further to get back into the same orientation relative to the Sun and 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 through its monthly cycle, the Earth has moved approximately 1/12th of the way around its orbit.

distant star

Sun

One sidereal month later

One synodic month later

New Moon

Follow this link to see a simulation showing the difference between the lunar sidereal and synodic periods.

This is similar to the distinction we made in the ActivityNight and Day between “sidereal day” and “mean solar day”.

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.

(c) Lunar Eclipses

When the Moon moves into the Earth’s shadow,a lunar eclipse occurs.

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

Sun Earth Moon

NOT TO SCALE!

While in the Earth’s shadow, the Moon is seen from Earth 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 passes through the atmosphere at the “edges” of the Earth on its way to the Moon:

Most of the spectrum is scattered away,leaving only the red light to 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 part of the spectrum is transmitted

Follow this link to see a simulation of a lunar eclipse.

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.

(d) Solar Eclipses

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

Although the Sun is much larger than the Moon, it is also much further away from us. By a convenient coincidence, the angular diameters of the Sun and Moon as seen from the Earth are almost the same – approximately 0.5°.

Remember that if you hold your hand out at arm’s length,then a finger’s width is roughly 1°,

… and the Sun and Moon haveangular diameter about half that.

(Check this out with the Moon,but not the Sun - looking directlyat the Sun, even for a very shorttime, can permanently damageyour eyesight.)

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

NOT TO SCALE!

Sun EarthMoon

The Moon’s shadow traces a path over the Earth:

to the Sun

and observers in the central dark part of the shadow(the umbra) see a total solar eclipse...

…while observers in the grey “rim” of the shadow (the penumbra) will see the Sun partly but not completely obscured - a partial solar eclipse.

to the Sun

Click on the links below to see a choice of two animations of a solar eclipse.

And follow this link to see a and click below to see NASA time-lapse images of the shadow of the Moon on the Earth’s surface, as seen from space.

Click here to see the animation

Click here to see animation 1

Click here to see animation 2

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 5° tilt of the Moon’s orbital plane limits the chances of the Sun, Moon and Earth being in alignment.

Sun Earth

Ecliptic plane

Plane of Moon’s orbit

5o

Moon above and belowplane of ecliptic

- no eclipse possible

NOT TO SCALE!

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

Follow this link to see a simulation illustrating the way the tilt of the Moon’s orbital plane limits the occurrence of solar eclipses.

Sun Earth

Ecliptic plane

Plane of Moon’s orbit

5o

Moon on ecliptic - solar and lunar eclipses

observed

NOT TO SCALE!

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

© Bill Ronald

If you follow this Internet link, you will reach NASA’s catalog of Solar Eclipse Paths from 1997 - 2030http://umbra.nascom.nasa.gov/eclipse/predictions/eclipse-paths.html

We’ll study more about features of the Sun such as sunspots, the chromosphere, corona and photosphere in the later Activities on the Sun.

NASA Photo NUMBER p-41508c: Image of the Earth and Moon from

Galileo http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg

NASA: View of Australia

http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_australia.jpg Gordon Garradd: Totally eclipsed Moonhttp://usrwww.mpx.com.au/~gjg/eclipse.htm copyright, reproduced with permission

NASA: Three Colour Filter Image of the Moon

http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg

NASA: Solar eclipse November 1994

http://umbra.nascom.nasa.gov/eclipse/images/hao_WLCC_941103.gif

Image Credits

Oliver Staiger: Partial Solar Eclipse images, Sept. 1997http://eclipse.span.ch/sept2.htm © Oliver Staiger, used with permission

Bill Ronald: Outer Corona (Total Solar Eclipse February 1998)

http://members.home.net/ronaldb/eclipse/1411e.jpg © Bill Ronald, used with permission

Lunar phaseshttp://www.googol.com/moon/

Image Credits

Now return to the Module 5 home page, and read more about lunar cycles in the Textbook Readings.

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