Chapter 2- Terrestrial Planet's and Earth's Moon

Constellation

An apparent pattern of stars grouped by ancient cultures, used for timekeeping and navigation.

Solstice

The longest or shortest day of the year; in the northern hemisphere, the summer solstice occurs around June 20-22.

Celestial Sphere

An imaginary sphere where stars are projected, with Earth's rotation and revolution perceived as the motion of the sphere.

North Celestial Pole (NCP)

projection of Earth’s north pole

South Celestial Pole (SCP)

projection of Earth’s south pole

Celestial Equator

projection of Earth’s equator to the sphere

Ecliptic

The path of the Sun across the celestial sphere, inclined 23.5 degrees to the celestial equator.

Earth’s Rotation

• when viewed form above the north pole, earth spins (rotates) counterclockwise on its axis

• form earth’s surface, every object seems to move across the sky once a day

• one rotation takes 24 hours

Celestial Coordinate System

angles measured with respect to the celestial equator are used like latitude and longitude on the Earth

• declination

• right ascension

Right Ascension

Angular distance eastward along the celestial equator from the point where the ecliptic crosses the equator, measured in hours.

Declination

The distance of an object north or south of the celestial equator, analogous to latitude, measured in degrees.

Horizon Coordinate System

unique to each location and time

• not a global coordinate system

Zenith

The point directly above an observer's location.

Meridian

line from north to south through the zenith, cutting the sky in half

Horizon

the circle 90 degrees away from the zenith

Nadir

point directly below observers location

Altitude

(latitude)- angle from horizon to object

Azimuth

angle from north measured east along horizon to the object

Circumpolar Stars

Stars that make circles around one of the celestial poles and never rise or set, visible at certain latitudes

• on north pole

• |omega**declination)| > 90^o - |L|

View at the Poles

standing at one of Earth'‘s poles, you would see half the celestial sphere

• a celestial pole is directly overhead

• stars rotate in 24 hours counter around NCP

• no star rises or sets: all circumpolar

View at the Equator

• all stars rise and set

• the celestial poles are on the northern and souther horizons

• observers can see the whole celestial sphere as it rotates

Coriolis Effect

The curving of objects' paths caused by the difference of Earth's rotation speeds at different latitudes.

Between the Poles and The equator

• elsewhere on earth, one pole is above the horizon

• the altitude of that pole is the same as the latitude

• some stars are circumpolar (never rise or set)

• some stars rise and set

• some stars are never visible

Celestial Navigation

• stars are useful for navigation

• latitude is equal to the celestial’s poles altitude

• longitude can be determined the time at which the sun passes overhead (crosses the meridian)

• requires accurate timekeeping devices

Earth’s Rotation: Why we dont receive our motion

• attributed motion to the celestial sphere

• Copernicus hupothesized that the earth was orbiting the sun in 1515

• in 1605, kepler suggested that the earth orbiting around the sun

• one of the key reasons people did not believe this theory was because they could not feel the motion

Effects of Earth’s Rotation

• objects on Earth’s surface move in a cricle once per day around earth’s rotation axis

• the closer an object is to the poles, the smaller the circle

• earths surface moves faster at the equator than at the latitudes

• projectiles will appear to curve to the right in the Northern Hemisphere and to the left in the Southern

Summer Solstice

The day of the year when the Sun is farthest north and the longest day occurs.

Autumnal Equinox

sun on the celestial equator, moving southward

Winter Solstice

sun farthest south, shortest day of the year

Vernal Equinox

sun on the celestial equator, moving northward

Artic/Antartic Circles

66.5 degrees N/S, where the sun is circumpolar on the first day of summer

Tropic of Cancer/Capricorn

the latitudes where the sun is directly overhead on the equinoxes

Lunar Eclipse

An eclipse that occurs when Earth is directly between the Sun and the Moon.

Solar Eclipse

An eclipse that occurs when the Moon is directly between the Sun and Earth.

Umbra

The small shadow cast by an object during an eclipse, where totality occurs.

Penumbra

The larger shadow cast around the umbra in an eclipse, where partial shadowing occurs.

Annular Eclipse

moon is not completely covering the sun: the moon is closer to the Earth

The moon turns red during a lunar eclipse. Why?

red light from the sun is bent through Earth’s atmosphere and other colors of light are scattered or absorbed

If the moon orbits the Earth every month, why don’t we see solar and lunar eclipses every month?

the moon’s orbit is tilted by 5.2 with respect to Earth’s orbit around the sun

Line of Nodes

The intersection of the plane of the Moon's orbit and the plane of Earth's orbit, significant for predicting eclipses.