Lunar Phases & Eclipses Astronomy 2012

The Moon orbits the Earth about once a “moonth” with one complete cycle of the lunar phases each month…

1-4 The Moon’s Phases 1. The rotation and revolution period of the Moon are equal and can be explained by the law of universal gravitation. 2. Rotation is the spinning of an object about an axis that passes through it. 3. Revolution is the orbiting of one object around another. 4. Phases of the Moon—the changing appearance of the Moon during its cycle—are caused by the relative positions of the Earth, Moon, and Sun.

Figure 1.24a: The Moon in various phases seen from above the Earth’s North Pole.

Figure 1.24b: Photos of the Moon in phases that correspond to the positions shown in Fig. 1-24a. (a-g) Photos courtesy of UCO/Lick Observatory

5. The phases follow the sequence of waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third (or last) quarter, waning crescent, new Moon. 6. Elongation is the angle of the Moon (or planet) from the Sun in the sky. 7. A sidereal revolution of the Moon takes about 27 1/3 days. 8. A sidereal period is the amount of time required for one revolution (or rotation) of a celestial object with respect to the distant stars.

9. A synodic period is the time interval between successive similar alignments of a celestial object with respect to the Sun. 10. A synodic revolution of the Moon takes about 29 1/2 days. 11. Lunar month is the Moon’s synodic period, or the time between successive phases: 29 d 12 h 44 m 2 s.9.

Figure 1.25: Sidereal vs synodic period

No really…the dark side and lunar rotation: A person standing in one spot on the moon would see the earth in the same part of the sky (directly overhead), unmoving. But, he/she would see the sun appear to move around the sky in about 29 earth days. The person would correctly deduce that the moon has a solar day of about 29 earth days. This person would also see that the stars appear to move around the sky in about 27 earth days. Again, the person would correctly deduce that the moon spins on its axis once in about 27 earth days. The moon definitely rotates. It just doesn't look like it, from where we stand. The moon rotates at a constant rate. But, its distance from earth, and speed in its orbit, varies quite a bit. This means that we can see a little ways around the edge of the moon. So, over a period of time, we can see slightly more than 50% of the moon, from earth.

You know from personal experience that we always see the same side of the moon…so it must rotate or we would see all sides as it orbited Earth.

The Moon rotates in the same period of time it takes to orbit the earth. Many moons in our solar system are gravitationally locked to their planet just like our moon is.

This also means that there is not really a dark side of the moon. Half of the moon is in darkness at any given time but that half changes throughout the month.

Lunar Eclipse Time Lapse Lunar Eclipse

Geometry of a Lunar Eclipse: Lunar eclipses occur when the Earth blocks sunlight from striking the Moon…

Lunar Eclipses are visible from most of the nighttime side of Earth. They are more frequent and widespread than solar eclipses because Earth’s shadow is larger than the moon’s…

1-5 Lunar Eclipses 1. Lunar eclipse is an eclipse in which the Moon passes into the shadow of the Earth. 2. Umbra is the portion of a shadow that receives no direct light from the light source. 3. Penumbra is the portion of a shadow that receives direct light from only part of the light source.

4. Eclipse season is a time of the year during which a solar or lunar eclipse is possible. 5. A lunar eclipse does not occur at each full Moon because the Moon’s plane of revolution is tilted 5° compared to the Earth’s plane of revolution around the Sun. Only during the two eclipse seasons that occur each year are the Earth and Moon positioned so that the Moon will enter the Earth’s shadow during a full Moon.

Types of Lunar Eclipses –1. Penumbral lunar eclipse is an eclipse of the Moon in which the Moon passes through the Earth’s penumbra but not through its umbra. –2. Total lunar eclipse is an eclipse of the Moon in which the Moon is completely in the umbra of the Earth’s shadow. –3. Partial lunar eclipse is an eclipse of the Moon in which only part of the Moon passes through the umbra of the Earth’s shadow. –4. A total eclipse of the Moon is never totally dark because some light is refracted toward the Moon by the Earth’s atmosphere. Most of this refracted light reaching the Moon is red; the blue portion has been scattered out.

Figure 1.26 abc: Total vs partial lunar eclipse

Figure 1.31: Shadows of Moon on Earth

Figure 1.29: Moon during eclipse Photo by Jim Rouse

Solar Eclipse…

Geometry of a Solar Eclipse…

Solar Eclipses only occur in the new moon phase and are visible only from a small section of the Earth. Typically in the tropics…

1-6 Solar Eclipses 1. Solar eclipse is an eclipse of the Sun in which light from the Sun is blocked by the Moon. 2. Total solar eclipse is an eclipse in which light from the normally visible portion of the Sun (the photosphere) is completely blocked by the Moon. 3. The corona—the outer atmosphere of the Sun—is visible during a total solar eclipse. Figure 1.32: Sun at total eclipse Courtesy of Alex York

The Partial Solar Eclipse –1.In a partial solar eclipse only part of the Sun’s disk is covered by the Moon. The Annular Eclipse –1. An annular eclipse is an eclipse in which the Moon is too far from the Earth for its disk to cover that of the Sun completely, so the outer edge of the Sun is seen as a ring or annulus. Figure 1.34: When the Moon is far away during a solar eclipse, the eclipse will be annular.