The Sky and Motions of the Earth, Moon, and Sun
Outline for Today • Configure and test PRS • History: Flat vs. Spherical earth • Mapping the Sky • Constellations • Motions in the Sky • Seasons • Precession of the Earth • Class discussion - Brains in Space • Phases of the Moon • Lunar and Solar Eclipses
Upcoming assignments: • Homework #1 due Tuesday at 9:00am • Must submit at least once! (not just save)
Setup menu ID:ABCDETF Entering your UserID • Turn on your PRS unit. • As soon as it begins scanning for classes, push the * button. This will halt the scanning process. • Push the up arrow once. You will see the following screen. • Push the green enter button to enter your UserID. up & down arrow buttons green enter button
ID: Enter Student ID Entering your UserID • Now you can enter your “Student ID”. This is the same as your PennState email address (not your 9-digit ID number). • Type a letter, and use the up & down arrows to scroll through the alphabet. • Type another letter to move to the second position. • Once you have entered all three letters, type in the numbers. • Once you have entered your ID, push the green enter button. Y Z T 5000 A B up & down arrow buttons green enter button
Testing the system Sample question: A) B) C) D) E)
Testing the system Sample question: A) B) C) D) E) PRS grading: -- counts under participation -- 1 point per class period for any entry -- 2 points per class period if >50% of answers are correct
Flat or Spherical Earth? • Proposed spherical earth: Aristotle (384-322 B.C.) • Evidence: curved shadow earth casts on the Moon • Additional evidence: new stars seen when sailing south • Spherical earth widely believed since time of Aristotle • Columbus set out to prove the world is round? Myth created by fictional biography of Columbus by Washington Irving
Constellations • Constellations are patterns of stars that the eye picks out. • The stars are usually not near each other; they just lie in the same direction. • Historically, the brightest stars are stars of the 1stmagnitude. The next brightest are stars of the 2nd magnitude. The faintest stars one can see by eye are 6th magnitude. (The biggest telescopes can reach m = 24 or so.)
Orion, the Hunter Scorpius, the Scorpion Leo, the Lion Cygnus, the Swan Antlia, the Air Pump Puppis, the Stern
Daily Motion • The earth’s axis of rotation also defines some places on the celestial sphere. • Thecelestial North poleis over the earth’s north pole. There happens to be a bright star near it, which is called Polaris, or the North Star. • Thecelestial equatoris the extension of the earth’s equator.
Daily Motion • Each day, the earth rotates once (west-to-east) on its axis. This causes us to face different directions and see different stars. The stars’ daily (diurnal) motion reflects the earth’s spin.
Yearly Motion In addition to rotating, the Earth alsorevolvesabout the Sun. As the Earth revolves, the Sun is projected in front of different constellations at different times of year. The path the Sun takes across the heavens is called the ecliptic.The constellations through which the Sun passes are thezodiac constellations. Because the Sun is bright, we can only see some constellations at certain times of year.
The Seasons • The earth’s spin axis is tilted 23.5° with respect to the plane of the earth’s orbit (the ecliptic). • Because of tilt, the “directness” of sunlight changes during the year. • Northern hemisphere tilted toward the Sun • summer solstice • Southern hemisphere tilted toward the Sun • winter solstice • Neither tilted toward the Sun • vernalandautumnal equinox • Seasons not caused by Earth changing its distance from the Sun!
Of course, some would object to this idea of a “summer” solstice in June and a “winter” solstice in December…
Precession In addition to its rotation and revolution, the earth’s axis alsoprecesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of thisprecession of the equinoxesis about 26,000 years.
Because of precession: • Polaris won’t always be the North Star • Orion won’t always be a winter constellation • Thuban was the North Star for the Pharaohs
Summary • Earth known to be round since Aristotle • Celestial sphere = Map of sky • Constellations = Patterns of stars on the sky • Diurnal (daily) motion = apparent motion of stars because of Earth’s rotation • Seasons: Caused by tilt of Earth’s spin axis, combined with revolution around the Sun • Precession = Slow wobble of earth as it spins, 26,000 years to complete a circuit
3rd quarter sunrise noon midnight sunset new full • Moon’s phase + position in sky Time 1st quarter
Phases of the Moon The Moon reflects light from the Sun, so its phase tells you the relative position of the Sun. You can therefore tell time from the Moon!
Phases of the Moon Full Moon occurs when the Moon is opposite in the sky from the Sun. This is calledopposition. New Moon occurs when the Moon is in the same direction as the Sun, i.e., inconjunction.
The Lunar Period The Moon moves west-to-east in the Sky (like the Sun), and takes about a month to circle the earth (hence the word month). But once again, there’s a difference between theMoon’s siderealperiod with respect to the stars (27 days), and thesynodicperiod with respect to the Sun (29 days).
Shadows of the Earth and Moon The Moon’s orbital plane is tilted by 5.2° from the ecliptic plane. Hence, 1/2 the time, the Moon is slightly North of the ecliptic (and 1/2 the time, it is south of the ecliptic). The shadow of one body (Earth or Moon) very rarely falls on the other.
Lunar Eclipses If the Moon crosses the ecliptic plane while exactly opposite the Sun, it will fall in the earth’s shadow. This is alunar eclipse. This happens roughly twice a year.
Solar Eclipses If the Moon crosses the ecliptic plane whileexactlyin front of the Sun, a tiny piece of the earth will fall in the Moon’s shadow. This is asolar eclipse. Since the Moon and Sun appear to be the same size in the sky, the alignment of atotal solar eclipsemust be perfect.
Partial Solar Eclipse If you’re on the edge of an eclipse path and only a slice of the Sun is blocked out, it’s called apartial eclipse.These are not very exciting, since the uneclipsed part of the Sun is still very bright, and the eye adjusts naturally to the slow changes in brightness.
Total Solar Eclipse You only get to see this outer atmosphere (corona) of the Sun if the Sun’s entire body is blocked out (total solar eclipse).
Annular Eclipses Because the Moon’s orbit about the earth is not perfectly circular, it is sometimes too far away to block out the whole Sun, even when perfectly aligned. When this happens, you get an annular eclipse. These are rarer than a total solar eclipse. Like other partial eclipses, unfortunately, they tend to be pretty boring.
Solar Eclipse Paths through 2020 (Buy your plane tickets now!)
Summary • Moon orbits Earth from West to East (same direction as Earth’s spin, and Earth’s revolution) • Phases of the Moon • Explanation via illumination by the Sun • Phase + sky position of the Moon = time of day • Period of phases (synodic, ~29 days) is longer than the Moon’s orbital period (sidereal, ~27 days) • Lunar and solar eclipses • Explanation via alignments of the three bodies • Don’t occur every month because Moon’s orbit is tilted relative to Earth’s orbit about Sun (ecliptic) • Lunar eclipes occur ~twice per year • Solar eclipses are much more rare (why?)