Astronomy Exam #1

Constellation

a group of stars that have defined boundaries looks like a particular shape in the sky and has been given a name.

Asterism

A recognizable group or pattern of stars that is not an official constellation

Zodiac

A band of 12/13 constellations along the celestial sphere that track the path of the Sun, Moon, and planets throughout the year.

Celestial Sphere

An imaginary giant sphere surrounding Earth, where all astronomical objects (stars, planets) appear to be projected

North Celestial Pole

The point in the sky directly above Earth’s Geographic North Pole where the northern sky appears to rotate.

South Celestial Pole:

The point in the sky directly above Earth’s Geographic South Pole where the southern sky appears to rotate.

Celestial Equator:

The projection of Earth’s equator into space onto the celestial sphere

• divides it into the Northern and Southern Hemisphere skies

Ecliptic:

The Sun’s apparent annual path across the celestial sphere, caused by Earth's orbit around it (The Sun).

Zenith

The point on the celestial sphere directly above an observer (90 degrees above the horizon).

Horizon:////

The boundary line where the Earth’s surface appears to meet the sky

• perpendicular to Zenith

Meridian: ////

An imaginary great circle in the sky that passes through the north and south celestial poles and is directly overhead through an observer's zenith

(*CS) Altitude:

The angular distance of a celestial object measured vertically from the observer’s horizon (from 0 degrees at the horizon to 90 degrees at the zenith)

(*CS) Azimuth:

The angular distance of a celestial object is measured horizontally along the horizon from due North (0 degrees) moving clockwise through East (90 degrees), South (180 degrees), and West (270 degrees)

Latitude vs Longitude

Latitude measures how far north or south a location is from the equator, while longitude measures how far east or west a location is from the prime meridian

Circumpolar:

Stars or constellations close enough to a celestial pole that they never drop below the local observer's horizon during daily rotation

• circumpolar stars never set

The Celestial Sphere notes

• The sky rotates about the celestial poles.

• The ecliptic is tilted 23.5⁰ with respect to the celestial equator.

Ptolemy and Almagest

Claudius Ptolemy is the Greek-Egyptian astronomer that wrote the almagest, a treatise of mathematical astronomy

• On apparent paths of planets and motions of the stars

Degrees Arcminutes and Arcseconds

• Degrees measure the apparent size of an object (see angular diameter) and its position on the celestial sphere.

• 1° is 60 arc minutes (‘), and 1′ is 60 arc seconds (“).

Polaris

• The current Northern star, finds with the pointer stars in the big dipper

• Polaris altitude = northern latitude

Precession

the observed gradual change in the rotational axis or orbit path of an astronomical body (ex, Earth)

• The North Star changes on a 26,000-year cycle.

• Now: Polaris, Ancient Egypt: Thuban

Total Lunar Eclipses and Opposition

Lunar eclipses occur when the Moon passes through Earth’s shadow

• They can only occur during a full moon when the Sun and Moon are at opposition.

• At totality, the Moon appears red.

• Everyone on Earth’s night side can see a lunar eclipse

Refraction vs Reflection

Refraction: The bending of light rays as they pass from one medium into another, most often from space into Earth’s atmosphere.

• Earth’s atmosphere refracts and scatters sunlight.

Reflection: when light from a star, is scattered or reflected off a neighboring dust cloud, making those dust cloud particles visible.

Partial Lunar Eclipse and Penumbral Lunar Eclipse

Partial: when only a portion of the Moon is blocked by Earth's (umbra)l shadow

Penumbral: when the Moon passes through Earth’s faint outer shadow—the penumbra (barely eclipsed)