The dark inner portion of the shadow cone is called the umbra. Solar and Lunar Eclipses Any opaque body illuminated by the Sun casts a shadow. This includes Earth and the Moon. A shadow cast by Earth or the Moon is shaped like a cone that points away from the Sun. The dark inner portion of the shadow cone is called the umbra. The lighter outer portion of the shadow is called the penumbra. These shadows always exist, but we only notice them during an eclipse. An eclipse in space occurs when one object moves into the umbra or penumbra of another object’s shadow. In other words, the object casting the shadow blocks the Sun’s light and keeps that portion of light from reaching the eclipsed object. The Sun, Moon, and Earth have to be aligned for an eclipse to occur. An eclipse within the system can be total or partial, depending on the degree to which the Sun, Earth, and Moon align.

Lunar Eclipse When Earth casts a shadow on the Moon, Earth blocks the Sun’s light from the Moon. The result is a lunar eclipse. A lunar eclipse can occur only when the Moon is in its full moon phase. The types of lunar eclipses that can occur include the following: A total lunar eclipse occurs when the Moon passes completely into the umbra, or total shadow of Earth. A partial lunar eclipse occurs when the Moon does not fully enter the umbra. A penumbral eclipse occurs when the Moon passes through only the penumbra, or partial shadow of Earth. Penumbral eclipses are hardly noticeable because the shadow on the Moon is very faint. Even when the Moon is completely in Earth’s umbra, not all light is cut off from the Moon. This is because light is refracted and filtered through Earth’s atmosphere and illuminates the eclipsed full moon. During totality, when the Moon is completely in Earth’s umbra, we can see the effects of this refracted light in the Moon’s very faint reddish-orange glow. The degree of color and brightness of the eclipsed Moon during totality depends on the conditions of Earth’s atmosphere at the time.

Solar Eclipse: The Sun is much larger and farther away from Earth than the Moon, but the sizes of the Sun and Moon sometimes appear to be the same, depending on the minor changes in distance based on their orbits. Remember, the Sun is 400 times larger than the Moon and also 400 times farther away from Earth. Therefore, the Moon is capable of blocking the apparent disk of the Sun from view. When the Moon comes between the Sun and Earth, the Moon blocks the Sun’s light and casts a shadow on Earth. The result is a solar eclipse. A solar eclipse can occur only during a new moon. Because of the relative distances of the Sun, Earth, and Moon from one another, and because of the size of the Moon’s shadow cone in relation to Earth’s diameter, a solar eclipse can only occur over a narrow band of Earth. These are the three types of solar eclipses: A total solar eclipse occurs if you are at a spot within the Moon’s umbra, a relatively narrow area on Earth. A partial solar eclipse is visible if you are just outside the Moon’s umbra but still in the penumbra (the lighter part of the Moon’s shadow). Only a portion of the Sun will be blocked and you will see a partial solar eclipse. A partial solar eclipse also may occur when the Moon is not quite in line with the Earth-Sun line and therefore cannot block all of the Sun from view. When this happens, only part of the penumbra reaches Earth and the umbra does not touch Earth at all, but is off to its side in space. If you are outside the penumbra, you see no solar eclipse at all. An annular eclipse occurs when the Moon is farthest from Earth in its orbit, resulting in the apparent size of the Moon being slightly less than that of the Sun. An outer ring of light from the Sun is visible around the Moon during an annular eclipse, and the tip of the Moon’s umbra falls just short of Earth, not reaching Earth at all.

Reflections Inquiry 6.1 A: A lunar eclipse occurs during a full moon. A lunar eclipse occurred on January 9. This was also the date of a full moon. B: A solar eclipse occurs during a new moon. A solar eclipse occurred on December 14. This was also the date of a new moon.

C: Three D: Two E: No, during 2001, a new moon and a full moon occurred every month, with two full moons in October. Lunar and solar eclipses each only occurred two to three times during 2001. F: A total solar eclipse occurs when the Sun, Moon, and Earth are perfectly aligned, and the Moon comes between the Sun and Earth.

G: A partial solar eclipse can occur when the Moon is slightly above or below the line of the Sun and Earth. H: An annular eclipse occurs when the Sun, Earth, and Moon align, but the Moon-in its elliptical orbit- is slightly farther away from the Earth than normal, causing the Moon to look slightly smaller than the Sun.

Reflections for Inquiry 6.2 A: Solar and lunar eclipses each occur about tow to three times a year. B: Solar and lunar eclipses do not occur each monthe because the Moon’s orbit is tilted, and the new moon and full moon are not always in line with the Sun and Earth. The Moon and Earth’s shadows normally fall above or below that line. Eclipses only occur when the new moon and full moon cross over Earth’s orbital plane-that is, when the Moon is very near the Sun-Earth line.

C: Solar and lunar eclipses seem to occur in “pairs,” with a lunar eclipse often occurring within 2-3 weeks of a solar eclipse, half the time it takes the Moon to revolve around Earth one time. These eclipse pairs occur within the same synodic month, when bothe the new moon and full moon are in line witht the Sun and Earth. Pairs of eclipses seem to occur about 6 months apart (positions B and D of inquiry 6.2). It is only during these times, roughtly 6 months apart, when the Sun-Earth line is approximately along the line of the Moon’s new and full phase.

D: The Moon’s orbit would have to be in the same plane as Earth’s orbit. In other words, for eclipses to occur each month, the Moon’s orbit could not be tilted.