Motions of the Earth and Sky Part I. Outline History: flat vs. spherical earth Map of the sky Constellations Diurnal and Yearly Motion The seasons Precession.

Motions of the Earth and SkyPart I

Outline

• 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 (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• Constellations are patterns of stars that have a historical reference to a mythological figure, animal, or object.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 meridian is the line in the sky passing overhead from north to south. The zenith is the point directly overhead. Noon is defined as the time for a local observer when the Sun crosses the meridian.A.M. = Ante Meridian (before the Sun crosses the meridian)P.M. = Post Meridian (after the Sun crosses the meridian)

The meridian, zenith, and horizon

An object here

appears on horizon

An object here

appears on horizon

An object here

appears at zenith

The Celestial Sphere = map of the sky

The celestial poles

The North celestial pole is the point in the sky above Earth’s north pole. There happens to be a bright star near it, which is called Polaris, or the North Star. Similarly, there’s a South celestial pole, but it’s not near a bright star.

If you are at the north pole, the north celestial pole is directly overhead. At the equator, the north & south celestial poles appear on the horizon.

Diurnal Motion

Each day, the earth rotates once (counterclockwise) on its axis. As a result, objects in space appear to move across the sky over the course of the day. This daily motion of the Sun, Moon, stars, and planets is called diurnal motion.

Diurnal Motion

During diurnal motion, stars in the northern half of the sky appear to move in circles around the north celestial pole (and southern stars circle the south celestial pole).

Yearly Motion

In addition to rotating, Earth also revolves about the Sun.

As viewed from Earth, the Sun appears in front of different constellations at different points in Earth’s orbit (i.e., different times of year). The path of the Sun across the stars is called the ecliptic. The constellations the the Sun moves across are the zodiac constellations.

Because of the Earth’s orbit around the Sun, some constellations are in the sky at night for only a part of the year.

The Seasons• Earth’s spin axis is tilted 23.5° from the plane of

Earth’s orbit.• Because of this 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 hemisphere is tilted toward the Sun vernal and autumnal equinox• Seasons not caused by earth changing its distance

from the Sun, or the fact that one hemisphere is physically close to the Sun than the other

At higher latitudes (closer to the poles), the Sun doesn’t reach as high in the sky as at lower latitudes. For instance, at the poles, the Sun never moves higher than 23.5 degrees above the horizon.

equator

N

S

equator

N

S

When one hemisphere is tilted toward the Sun (summer in that hemisphere), the Sun reaches higher in the sky and is in the sky for a longer period of time (i.e., more hours of daylight) than for the other hemisphere (which is in winter).

PrecessionIn addition to its rotation and revolution, the earth’s axis also precesses (wobbles) like a top. The angle between the spin axis and the earth’s orbital plane remains at 23.5°, but the direction of the spin axis changes. It takes 26,000 years for the spin axis to complete one full wobble.

Precession

• 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:

http://www.csulb.edu/~gpickett/ps112_ems.html

http://www.astro.psu.edu/users/kluhman/a5/Lunar_Nav.swf

Phases of the Moon

The Moon experiences phases because during its orbit around the Earth, the relative positions of the Moon and Sun change. As a result, we see different amounts of the illuminated and dark halves of the Moon.It takes roughly 28 days for the Moon to orbit the Earth once, and hence this is the time between one full moon and the next full moon.

sunrise

sunset

noon midnight

new

3rd quarter

full

1st quarter

Moon’s phaseMoon’s position in skyLocal time

Given 2 of these, you can deduce the 3rd

A celestial object is in opposition if it appears on the opposite side of the sky from the Sun. For instance, the Moon is in opposition when it is full.

An object is in conjunction when it is near the Sun in the sky. The Moon is in conjunction when it is new.

Opposition/conjunction refer to the sky from where one is standing. This is usually on Earth, but one can also consider opposition/conjunction as viewed from other places in the solar system.

Opposition and Conjunction