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 5 o relative to the ecliptic.

As we watch the Moon in the sky 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 partly on the Moon.

However once the relative positions of the Earth, Sun and Moon during the phases are taken into account, it is clear that the phases of the Moon are due to our viewing varying proportions of sunlight & shadow from the Sun during each month.

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

On the Internet, you can use John Walker’s “Earth and Moon viewer” site, to see how the Moon would look now both from the Earth and from the Sun. It also shows how the Earth would look now from the near side of the Moon. Note that the far side of the Moon is not necessarily the dark side of the Moon.

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 um/mooncal/coinrotationani2.gif

The period of 27.3 days, called the sidereal period - is relative to the “fixed stars”. Earth This means that the Moon goes from one particular position 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 through its monthly cycle, the Earth has moved approx. 1/12th of the way around its orbit.

This is similar to the distinction we made in the Activity on Night 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.

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 from Earth as darkened and somewhat reddened. The reddening effect comes about for the same reason that sunsets are red:

The Earth’s atmosphere scatters much of the Sun’s light, but the red end of the spectrum As the Sun’s light takes its long path through the atmosphere at the “edges” of the Earth:

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 end of the spectrum to be transmitted through the atmosphere:

… and the reddish light faintly illuminates the Moon while 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 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.

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. 0.5 o. Solar Eclipses only occur with a new Moon

Solar eclipses 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 o, 1o1o … 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 & 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.

Follow link (/essmovs/h1.htm) to see an animation of a solar eclipse,link (/essmovs/h1.htm) And follow this link to see a and follow this link (/essmovs/h19.htm) to see NASA time-lapse images of the shadow of the Moon on the Earth’s surface, as seen from space.link (/essmovs/h19.htm)

Here are three images of a partial solar eclipse taken from 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 o 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

Apogee and Perigee of the Moon Apogee and perigee refer to the distance from the Earth to 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: 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:

If you follow this Internet link, you will reach NASA’s catalog of Solar Eclipse Paths from

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 NASA: View of Australia Gordon Garradd: Totally eclipsed Moon copyright, reproduced with permission NASA: Three Colour Filter Image of the Moon NASA: Solar eclipse November Image Credits

Oliver Staiger: Partial Solar Eclipse images, Sept © Oliver Staiger, used with permission Bill Ronald: Outer Corona (Total Solar Eclipse February 1998) © Bill Ronald, used with permission

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