Equinoxes and Solstices When the ecliptic and celestial equator intersect, day and night are each 12 hours long: the equinox. When the Sun reaches its most northern and southern points in the sky: the solstice.

The Sun appears to move on the celestial sphere. The Sun appears to rise and set at different locations: Winter: toward the south Summer: toward the north Fall and Spring: due east and west

ConceptCheck If Earth’s axis were not tilted, but rather was straight up and down compared to the path of Earth’s orbit, would observers at Earth’s North Pole still observe periods in which the Sun never rises and the Sun never sets? How long does the Sun take to move from being next to a bright star all the way around the celestial sphere and back to that same bright star? How often each year does an observer standing on Earth’s equator experience no shadow during the noon- time Sun? Approximately how many days are there between the northern solstice and the March equinox?

The Moon is lit by sunlight. Just like the Earth, half of the Moon is lit by sunlight. The Moon does not produce its own light. This image of the Earth and Moon was taken by the Galileo spacecraft.

Understanding the Moon’s Phases The phase of the Moon is a result of our point of view.

The “pictures” of the Moon show what you would see from Earth when the Moon is in that location.

The Moon’s Synchronous Rotation The Moon makes one orbit around Earth and spins one time on its axis in the exact same amount of time. We always see the same side of the Moon―not the “dark side,” but the “far side.”

Sidereal and Synodic Months The Moon orbits the Earth as Earth orbits the Sun, so the Moon has to “catch up.” The Moon returns to its place on the background stars: sidereal month. The Moon completes a full cycle of phases: synodic month.

ConceptCheck If an observer on Earth sees just a tiny sliver of the crescent moon, how much of the Moon’s total surface is being illuminated by the Sun? If the Moon appears in its waxing crescent phase, how will it appear in two weeks? If astronauts landed on the Moon near the center of the visible surface at full moon, how many Earth days would pass before the astronauts experienced darkness on the Moon? If Earth were orbiting the Sun much faster than it is now, would the length of time between full moons increase, decrease, or stay the same?

Eclipses occur when the Sun, Moon, and Earth are perfectly aligned. The Moon’s orbital plane is just a little off the ecliptic.

The Sun, Moon, and Earth rarely line up.

Lunar Eclipses When the Moon is opposite the Sun, it can travel through the Earth’s shadow. The Earth’s shadow is complete in the center and partial on the edge.

Total Lunar Eclipse from January 20, 2000

Total Solar Eclipses The Moon totally covers the face of the Sun. From inside the darkest part of the Moon’s shadow. Those inside of the Moon’s partial shadow see a partial eclipse.

Spectacular, Rare Total Eclipses

Annular Solar Eclipses When the Moon is at its farthest position, the cone of its shadow doesn’t reach Earth. The Moon appears to be too small to cover the Sun.

ConceptCheck Why don’t lunar eclipses occur each time the Moon reaches full moon phase? Why does the eclipsing Moon spend more time in the penumbral shadow than in the umbral shadow? Why can total lunar eclipses be seen by people all over the world, whereas total solar eclipses can only been seen from a very limited geographic location? If you had a chance to observe a total solar eclipse and a total lunar eclipse, in general, how much longer would you expect one type to last than the other?

Next Chapter: Chapter 2 Decoding the Hidden Messages in Starlight