ASTRONOMY Ch. 3

lunar eclipse

A lunar eclipse occurs when the Moon moves into and out of Earth's shadow. Note the curved shape of the shadow—evidence for a spherical Earth that has been recognized since antiquity.

Many people incorrectly guess that the phases of the Moon are caused by Earth's shadow falling on the Moon. How would you go about convincing a friend that the phases of the Moon have nothing to do with Earth's shadow? Describe the observations you would use to show that Earth's shadow couldn?t be the cause of phases.

Earth's shadow cannot cause the phases of the Moon because the phases result from two basic facts: -Half the ball always faces the Sun and is bright while the other half always faces away from the Sun and is dark -As you look at the ball in different orbital positions, you see different combos of its bright and dark faces.

Briefly describe the Moon's cycle of phases. Can you ever see a full moon at noon? Explain.

New Moon, Waxing crescent, first quarter, Waxing gibbous, full, Waning gibbon, third quarter, and Waning crescent. Yes. After about one week, the Moon is one-quarter of the way around its orbit, at the first quarter phase. Half of the Moon's illuminated side is visible to Earth observers. Because of its eastward motion, the Moon now lags about one-quarter of the day behind the Sun, rising around noon and setting around midnight.

If there is going to be a total lunar eclipse tonight, then you know that

The Moon's phase is full.

line of nodes

The line along which the plane of the Moon's orbit intersects the plane of the ecliptic.

Why do we always see the same side of the moon?

The moon is always rotating on its axis; the time it takes to rotate once is exactly the same length of time it takes for the moon to orbit the earth. We always see the same side of the moon, the near side.

This morning, I saw the full moon setting about the same time the sun was rising.

The statement is true. The earth during a full moon is on the opposite end of the horizon compared to the moon. This causes the setting of the moon, and the rising of the sun.

Why don't we see an eclipse at every new and full moon? Describe the conditions needed for a solar or lunar eclipse.

We don't see an eclipse at each new moon or full moon because eclipses only occur when the moon is in perfect alignment with the earth and sun while passing through the nodes. For a lunar eclipse, the earth has to lie directly between the sun and the moon, casting it's shadow on the moon. For a solar eclipse, the moon has to lie directly between the sun and the earth so as to cast it's shadow on the earth.

Ecliptic

a great circle on the celestial sphere representing the sun's apparent path during the year, so called because lunar and solar eclipses can occur only when the moon crosses it.