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 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 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 the shadow of the Earth falling on the Moon.

However, once the relative 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 as seen from Earth Phases of the Moon Third quarter A B C D E F G H Sun’s rays New moon Full moon First quarter A B C D E F G H Phases as seen from Earth

It also shows how the Earth would look from the near side of the Moon. 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 near side of the Moon. (Note that the far side of the Moon is not necessarily the dark side of the Moon.)

(b) Lunar Sidereal & Synodic Periods The Moon orbits the Earth with a period of 27.3 days, always with the same face turned towards us.

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 particular position 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 synodic period is the interval between identical lunar phases. 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.

- 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 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! One synodic month later One sidereal month later New Moon

This is similar to the distinction we made in the Activity Night and Day between “sidereal day” and “mean solar day”. Follow this link to see a simulation showing the difference between the lunar sidereal and synodic periods.

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 The Moon’s phases are not due to shadowing by the Earth, but lunar eclipses are. When the Moon moves into the Earth’s shadow, a lunar eclipse occurs. 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 reason that sunsets are red:

As the Sun’s light passes through the atmosphere at the “edges” of the Earth on its way to the Moon: The Earth’s atmosphere scatters much of the Sun’s light, but the red end of the spectrum

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

The Earth’s atmosphere scatters much of the Sun’s light, but some of the red part of the spectrum is transmitted … and the reddish light faintly illuminates the Moon while it is within the Earth’s shadow. Follow this link to see a simulation of a lunar eclipse. What would you expect to see if you were on the Moon during 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 ring of reddish-tinged atmosphere.

(d) Solar Eclipses 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°. Because of this coincidence, we are occasionally treated to one of nature’s most spectacular events - a total solar eclipse.

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 have angular diameter about half that. (Check this out with the Moon, but not the Sun - looking directly at the Sun, even for a very short time, can permanently damage your eyesight.)

When the Moon moves directly (or almost directly) between the Sun and the Earth, a solar eclipse occurs. NOT TO SCALE! Sun Earth Moon

to the Sun The Moon’s shadow traces a path over the Earth: and observers in the central dark part of the shadow (the umbra) see a total solar eclipse...

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

And follow this link to see a Click on the links below to see a choice of two animations of a solar eclipse. Click here to see animation 1 Click here to see animation 2 and click below to see NASA time-lapse images of the shadow of the Moon on the Earth’s surface, as seen from space. And follow this link to see a Click here to see the animation

Here are three images of a partial solar eclipse taken from Kangaroo Island, South Australia on 2 Sept, 1997 (The red circle is highlighting the location of a sunspot) © Dan Staiger

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. Moon above and below plane of ecliptic - no eclipse possible Plane of Moon’s orbit Ecliptic plane Sun Earth 5o NOT TO SCALE!

Moon on ecliptic - solar and lunar eclipses observed 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. Moon on ecliptic - solar and lunar eclipses observed Plane of Moon’s orbit Ecliptic plane Sun Earth 5o NOT TO SCALE! Follow this link to see a simulation illustrating the way the tilt of the Moon’s orbital plane limits the occurrence of solar eclipses.

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 - 2030 http://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.

Image Credits 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 Moon http://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. 1997 http://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 phases http://www.googol.com/moon/

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