Motions of the Earth and Sky I

Outline for Today History: flat vs. spherical earth Map of the sky Constellations Diurnal and Yearly Motion The seasons Precession Phases of the Moon

Flat or Spherical Earth? Proposed spherical earth: Aristotle ( 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Constellations Constellations are patterns of stars that have a historical reference to a mythological figure, animal, or object. The stars are usually not near each other in space; they just lie in the same direction.

124 LY 101 LY ~80 LY

Ursa Major, The Big Bear (Big Dipper) The Big Dipper is an asterism within the constellation of Ursa Major. An asterism is a prominent pattern of stars that is smaller than a constellation.

The Celestial Sphere = map of the sky

Daily Motion The earth’s axis of rotation also defines some places on the celestial sphere. The celestial North pole is over the earth’s north pole. There happens to be a bright star near it, which is called Polaris, or the North Star. The celestial equator is 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.

stars appear to rotate about the north & south celestial poles

Yearly Motion In addition to rotating, the Earth also revolves about 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 heavens is called the ecliptic. The constellations which the Sun passes through are zodiac 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 is tilted toward the Sun  vernal and autumnal equinox Seasons not caused by earth changing its distance from the Sun

The Seasons During summer, the Sun moves higher in the sky, so sunlight is more direct, and it is visible for a longer period of time.

Precession In addition to its rotation and revolution, the earth’s axis also precesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about 26,000 years.

Precession In addition to its rotation and revolution, the earth’s axis also precesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about 26,000 years.

Polaris won’t always be the North Star The season during which a constellation appears at night changes very slowly over time Because of precession:

sunrise sunset noonmidnight Moon’s phase + position in sky  Time new 3rd quarter full 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 called opposition. New Moon occurs when the Moon is in the same direction as the Sun, i.e., in conjunction.