ASTRONOMY 161 Introduction to Solar System Astronomy

Astronomy 161: The Web Page www.astronomy.ohio-state.edu/~depoy/Astro161/astro161.html Astronomy Picture of the Day (APOD): http://antwrp.gsfc.nasa.gov/apod/

The Celestial Sphere Friday, January 5

The Celestial Sphere: Key Concepts (1) The sky as seen from Earth is divided into 88 constellations. (2) It is convenient to pretend the stars are attached to a celestial sphere. (3) The celestial sphere appears to rotate about the celestial poles (1 day). (4) The Sun appears to move west to east relative to stars (1 year). (5) The Moon appears to move west to east relative to stars (1 month).

(1) The sky is divided into 88 constellations Mesopotamia, circa 3000 BC: oldest know constellations Ptolemy, 2nd century AD: 48 constellations in northern sky 16th to 18th century AD: unmapped regions of sky filled in

Constellations are largely arbitrary Other cultures, other constellations: Example: Ursa Major = bear, dipper, bull's leg, grain scoop, wagon, plow, etc. Stars in a constellation usually are not at the same distance from us. At a different place in our Galaxy, we would see different star patterns.

A modern star chart of Ursa Major:

(2) Stars are “attached” to a celestial sphere Distances to stars are hard to measure. However, we can pretend all stars are at the same distance from us, attached to a large celestial sphere. Position on the celestial sphere is known even when the distance in unknown.

Celestial Sphere: A large imaginary sphere centered on Earth

Special locations on the celestial sphere North Celestial Pole = point directly above Earth’s North Pole (near the star Polaris) South Celestial Pole = point directly above Earth’s South Pole (no nearby bright star) Celestial Equator = circle directly above Earth’s Equator

Distances between points on the celestial sphere are measured in degrees, arcminutes, and arcseconds 360 degrees in a circle 60 arcminutes in a degree 60 arcseconds in an arcminute ½ degree = angular size of Sun & Moon

Celestial navigation made simple At Earth’s North Pole: Polaris is directly overhead At Earth’s Equator: Polaris is due north, on the horizon In Earth’s Northern hemisphere: Polaris is due north - height above the horizon (in degrees) is equal to your latitude (in degrees)

(3) The celestial sphere appears to rotate about the celestial poles (1 day cycle) Observation: Stars, Sun, Moon and planets move in counterclockwise circles around north (south) celestial pole. Objects near the celestial equator move east to west when above the horizon (“rising” in east, “setting” in west). What causes these circular motions?

Explanations of the 1 day cycle: HYPOTHESIS #1 (Ptolemy, 2nd century): Earth is stationary; stars are attached to a sphere that revolves around the Earth once per day. WRONG! HYPOTHESIS #2 (Copernicus, 16th cent.): Stars are stationary; Earth rotates about its axis once per day. RIGHT!

Nicolaus Copernicus (1473-1543)

(4) The Sun appears to move west to east relative to stars (1 year cycle) Today the Sun is “in” a particular constellation, next month in a different one, etc. Sun’s path on the celestial sphere = ecliptic Constellations through which the ecliptic runs = zodiac The ecliptic is NOT the same as the celestial equator!

Observation: Sun moves west to east relative to stars (about 1 degree per day). What causes this annual motion?

Explanations of 1 year cycle: HYPOTHESIS #1 (Ptolemy): Sun revolves around Earth at a slightly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 (Copernicus): Earth revolves around the Sun, once per year. RIGHT!

(5) The Moon appears to move west to east relative to stars (1 month cycle) Today the Moon is “in” Virgo In two weeks: Pisces In four weeks: Virgo, again. Observation: Moon moves west to east relative to stars, taking 27.3 days to complete cycle. What causes this monthly motion?

Explanations of 1 month cycle: HYPOTHESIS #1 (Ptolemy): Moon revolves around Earth at a significantly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 (Copernicus): Moon revolves around Earth, once per month. RIGHT!