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
Motions of the Earth, Moon, and Sun
Entering your UserID
up & down
green enter button
Enter Student IDEntering your UserID
up & down
green enter button
-- counts under participation
-- 1 point per class period for any entry
-- 2 points per class period if >50% of answers are correct
Myth created by fictional biography of Columbus by Washington Irving
Scorpius, the Scorpion
Leo, the Lion
Cygnus, the Swan
Antlia, the Air Pump
Puppis, the Stern
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.
Of course, some would object to this idea of a “summer” solstice in June and a “winter” solstice in December…
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.
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!
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 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).
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.
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.
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.
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.
You only get to see this outer atmosphere (corona) of the Sun if the Sun’s entire body is blocked out (total solar eclipse).
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.
(Buy your plane tickets now!)