Astronomy Unit 2 The Moon 1
Traveling on Spaceship Earth Although we imagine ourselves to be at rest, the Earth takes part in the motions outlined below. The Earth spins about its N-S axis, with a period of 1 day, and a rotational speed varies from 1650 km/hr (1030 mi/hr) at the equator to zero at the poles. The Earth orbits the Sun with a 1 year period, and a speed of above 100,000 km/hr (60,000 mi/hr). Our solar system orbits the center of our galaxy with a 230 million year period, and a speed slightly of about 800,000 km/h (500,000 mi/hr). Our galaxy orbits the mass-center of the Local Group of galaxies, which in turn orbits the center of the Local (or Virgo) Supercluster. 2
Siderial and Synodic Periods A siderial period is a period measured with respect to the distant stars. A synodic period is the period measured from a planet (or moon). The solar day is the synodic day measured from Earth, which is longer than the siderial day by about 4 min. The lunar month is the synodic month measured from Earth, which is longer than the siderial month by approximately 2.2 days. The tropical year is the synodic year, measured between successive vernal equinoxes, which is shorter than the siderial year by about 20 minutes. 3
Solar and Sidereal Days The solar day is the average time (24 hours) between successive noon-times, as measured at 0 o longitude in Greenwich, England (the prime meridian). The sidereal day is the time (23 hours 56 min.) taken for a planet to make one complete revolution. 4
Lunar and Sidereal Months The synodic or lunar month is the time (approximately 29½ days) between identical phases of the moon; e.g. from full moon to full moon. The sidereal month is the time (approximately 27.3 days) it takes the Moon to make one full orbit (360 o ) around the Earth. 5
The Year and the Calendar Ancient astronomers realized that the year was roughly 365¼ days long. In 47 BCE, Julius Caesar added an extra day every 4 years, thus creating leap years of 366 days. Pope Gregory XIII reformed the Julian calendar in 1582, leaving out 10 days to get the seasons back on schedule, and decreeing that only those century years divisible by 400 were to be leap years. The average Gregorian year differs by only one day in 3300 years from the tropical year. With the modification that the years 4000, 8000, 12,000 and 16,000 are not to be leap years, the Gregorian system will not have to be revised for 20,000 years. An extra second was added between Dec. 31, 2008 and Jan.1, 2009 to allow for irregularities in the Earth s rotation. 6
Lunar Calendars Lunar calendars follow the Moon s cycle, which averages 29½ days per month. Since the year would contain only 12 x 29.5 = 354 days, an additional month was added usually every 3 years. The Jewish calendar (now in the year 5767) is lunar, and is synchronized with the solar calendar by following the 19 year cycle, introduced by the Greek astronomer Meton in 432 BCE. Easter has a partially lunar basis, being scheduled as the first Sunday following the first full moon on or after March 21. The Islamic calendar is purely lunar, so that 12 months contain about 11 days fewer than a solar year. That is why, for example, Islamic festivals, such as Ramadan begin about 11 days earlier on each subsequent year. 7
Phases of the Moon 1 8
Sky at Sunset The Moon s position at sunset is shown for 14 evenings, beginning at the new moon and ending at the full moon. Note that west is to your right, which occurs if you are facing to the south, so that the Sun sets to your right. 9
Sky at Sunrise The Moon s position at sunrise is shown for 14 evenings, beginning at the full moon and ending at the new moon. Note that west is to your right, which occurs if you are facing to the south, so that the Sun sets to your right. 10
PHASES OF THE MOON PHASE SHAPE MOONRISE WHEN VISIBLE AT NIGHT New moon - dawn - Waxing crescent late morning evening 1 st quarter noon before midnight Waxing gibbous afternoon until early morning Full moon dusk All night Waning gibbous evening from evening 3 rd quarter midnight after midnight Waning crescent early morning early morning
Solar and Lunar Eclipses 1 The plane in which the Moon orbits the Earth makes an angle of 5.2 o with plane of the ecliptic. For an eclipse to occur, the Moon must be full or new at the same time as its path crosses the ecliptic. 12
Solar and Lunar Eclipses 2 The line of nodes is a hypothetical line joining the two points at which the Moon s orbit crosses the ecliptic. Eclipses occur when the line of nodes points towards the Sun. 13
The Eclipse Seasons Eclipses are relatively rare, because for eclipses to occur, the Moon must be full or new, just as it crosses the ecliptic plane. There are just two short periods in a year, known as the eclipse seasons, when eclipses can occur, although there is no guarantee of eclipses occurring during a particular season. Between 2 and 5 solar eclipses can occur in a year, and a similar number of lunar eclipses. However, the total number of eclipses in a year cannot exceed 7. It was known to ancient astronomers that the basic pattern of eclipses repeats every 18 years 11.3 days. This repetition pattern is known as the Saros cycle. 14
Solar Eclipses 1 A solar eclipse occurs when the Moon blocks some or all of the Sun s light, so that the Moon s shadow falls on the Earth. The umbra, the central region of the Moon s shadow, is surrounded by the penumbra. Only in the umbra is the sunlight totally blocked, so that a total solar eclipse or an annular solar eclipse occurs. A total solar eclipse occurs when the Moon is relatively close to the Earth, so that it appears large enough to totally blot out the Sun, thus allowing the faint solar corona to be seen. An annular solar eclipse appears as a thin ring encircling the Moon s disk when the Moon is too far from the Earth for it to totally block out the Sun. 15
Solar Eclipses 2 The umbra forms a dark spot, which is the region of the total or annular eclipse. 16
Solar Eclipses 3 A total eclipse occurs when the Moon is close enough to block out the Sun s surface, allowing its outermost layer the corona to be seen. A partial eclipse is seen from the shadow given by the Sun s penumbra. An annular eclipse occurs when the Moon is far enough away, so that it cannot hide the Sun s surface completely. 17
Total Solar Eclipse Only during a total solar eclipse is the solar corona visible. 18
Annular Eclipse 19
Solar Eclipse Tracks 2000-2020 The width of the track depends both on the Earth s latitude and the distance of the Moon from the Earth during the eclipse. Saros cycle 20
Lunar Eclipses 1 A lunar eclipse occurs when the Moon enters the Earth s shadow. 21
Lunar Eclipses 3 The Moon looks red during a total lunar eclipse for the same reason that the Sun appears reddish at sunrise and sunset, and the sky appear blue. Sunlight is composed of all the colors of the rainbow (red, orange, yellow, green, blue, violet), and the Earth s atmosphere preferentially scatters the blue end of this spectrum of colors. The scattered blue light gives the sky its color, while the missing blue end of the spectrum makes the Sun appear yellow during the day and red at sunrise and sunset, when the Sun s rays take a longer path through the atmosphere to reach us. 22
Lunar Eclipses 2 The Moon appears red in a total lunar eclipse because of the preferential scattering by the Earth s atmosphere of the blue end of the spectrum of colors in sunlight. As a result, more of the Sun s red light reaches the Moon. 23
Lunar Eclipse Over Dallas 24