Chapter 9

The Moon

Why does the Moon keep the same face turned toward the Earth

• Revolution and rotation - 182– The moon moves eastward 13 degrees per day, it also

moves east in relation to the sun…which means that moonrise and moonset grow later throughout the month

• The Month - 182– It takes the moon 29.53 days to go through the lunar

phases and return to the starting position relative to the sun (synodic)

– It takes the moon 27.32 days to return to position relative to the stars, this would be one complete orbit around the earth (sidereal)

Figure 09_01

Why does the Moon keep the same face turned toward the Earth

• Synchronous Rotation - 182– The moon revolves around the earth in 27.32 days, the

moon rotates on it axes every 27.32 days…This means we always see the same side of the because it is always turning towards the earth

– If you lived on the moon and lived on the near side, the earth would go through phases and be in the same location of the sky all the time.

– If you lived on the far side, you would never see the earth.

– Daytime would last 13.66 days on the moon

Figure 09_02a

Figure 09_02b

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• The orbit and size of the moon - 183– The moon has an elliptical orbit, with an average

distance of 384,400 km…Perigee is 356,400 km and apogee is 406,700 km

– The moon obits on the ecliptic plane and since the earth is tilted in its orbit, this means the moon crosses north (ascending node) and south (descending node) of our equator

– Refer to table 9.1 on page 184

Figure 09_04

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Eclipses – 184– It can take an hour for the moon to cover the sun, as the

total eclipses approaches and the moon covers the sun Baily’s Beads appear

– Then finally the diamond ring as the moon completely covers the sun, then the total phase of the eclipse has begun

– Then all that is seen is the Sun’s Corona, it may even be possible to see a prominence or two

– Totality never last longer then 7.66 minutes

Figure 09_06

Figure 09_07

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Eclipses as omens – 186– First lunar eclipse was record by the Mesopotamians

4000 years ago, the Chinese record the first Solar eclipse a few hundred years later

– People would amazed and feared such events to the point that it saved peoples lives, changed the outcomes of wars, and in some case changed politics

– Even people who could predict eclipses would not completely understand them

Figure 09_10

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Eclipses and shadows – 186– The umbra is the inner part of the shadow that

completely covers the sun– Penumbra only offers partial coverage of the sun– In the earth’s penumbra though light is still reflected

onto the moon, light is bent to the red– an annular eclipse is when the sun appears as a ring

Figure 09_12a

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Eclipses and Lunar Phases – 187– Eclipses only occur when the earth, sun and moon are in

perfect alignment. Therefore eclipses can only occur during a full moon or new moon phase

– A solar eclipse occurs when the earth passes through the moon’s shadow. (new moon)

– A lunar eclipses occurs when the moon passes through the earth’s shadow. (full moon)

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Umbral shadows and eclipses tracks– (lunar Eclipse)The earth’s umbral is 1,380,000 km in

length, 9,200 km across and extends beyond the moons orbit. The moon takes 3 hours to travel through it and is in totality for about 100 minutes

– (Solar Eclipse)The moons umbral is 371,000 km (average distance of 384,400 km…Perigee is 356,400 km) at largest 270 km across, people on the eclipses track will see a total eclipses

– Longest eclipses is 7min 40 sec….average is 2 to 3 minutes…covers 10,000 km and 150 km wide

What causes eclipses of the Sun and the Moon?How can we predict eclipses?

• Predicting eclipses– The moon must be near a node to cause an eclipse,

therefore the node must point toward the sun….eclipse season last for one month and happens every 5.7 months…the most you can have in a year is 7

– Every 18 years 11.33 days the same eclipse will happen…but is shift .33 the distance on the earth surface...which means 54 years 1 month for it to happen at the same location

– Page 192 figure 9.16