4The 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).
5(1) The sky is divided into 88 constellations Mesopotamia, circa 3000 BC: oldest know constellationsPtolemy, 2nd century AD: constellations in northern sky16th to 18th century AD: unmapped regions of sky filled in
6Constellations 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.
9(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.
10Celestial Sphere: A large imaginary sphere centered on Earth
11Special 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
12Distances between points on the celestial sphere are measured in degrees, arcminutes, and arcseconds 360 degrees in a circle60 arcminutes in a degree60 arcseconds in an arcminute½ degree = angular size ofSun & Moon
14Celestial navigation made simple At Earth’s North Pole: Polaris is directly overheadAt Earth’s Equator: Polaris is due north, on the horizonIn Earth’s Northern hemisphere: Polaris is due north - height above the horizon (in degrees) is equal to your latitude (in degrees)
15(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?
20Explanations 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!
22(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 = eclipticConstellations through which the ecliptic runs = zodiacThe ecliptic is NOT the same as the celestial equator!
24Observation: Sun moves west to east relative to stars (about 1 degree per day). What causes this annual motion?
25Explanations 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!
26(5) The Moon appears to move west to east relative to stars (1 month cycle) Today the Moon is “in” VirgoIn two weeks: PiscesIn 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?
27Explanations 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!