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The Celestial Sphere 1 The Celestial Sphere • Geocentrially speaking, the Earth sits inside a celestial sphere. Fixed on the sphere are stars. 2 Constellations • Constellations are patterns of stars that the eye picks out. The stars are usually not near each other; they just lie in the same direction. • Historically, the brightest stars are stars of the 1st magnitude. The next brightest are stars of the 2nd magnitude. The faintest stars one can see by eye are 6th magnitude. (The biggest telescopes can reach m = 26 3 or so.) Orion, the Hunter Cygnus, the Swan Scorpius, the Scorpion Antlia, the Air Pump Leo, the Lion 4 Puppis, the Poop Deck Constellations Andromeda (Princess) Circinus (Compasses) Lacerta (Lizard) Piscis Austrinus (Southern Fish) Antlia (Air Pump) Columba (Dove) Leo (Lion) Puppis (Poop Deck) Apus (Bird of Paradise) Coma Berenices (Bernice’s Hair) Leo Minor (Little Lion) Pyxis (Compass Box) Aquarius (Water Bearer) Corona Australis (Southern Crown) Lepus (Hare) Reticulum (Net) Aquila (Eagle) Corona Borealis (Northern Crown) Libra (Scales) Sagitta (Arrow) Ara (Altar) Corvus (Crow) Lupus (Wolf) Sagittarius (Archer) Aries (Ram) Crater (Cup) Lynx (Lynx) Scorpius (Scorpion) Auriga (Charioteer) Crux (Southern Cross) Lyra (Lyre) Sculptor (Sculptor) Boötes (Herdsman) Cygnus (Swan) Mensa (Table) Scutum (Shield) Caelum (Engraving Tool) Delphinus (Dolpin) Microscopium (Microscope) Serpens (Serpent) Camelopardalis (Giraffe) Dorado (Swordfish) Monoceros (Unicorn) Sextans (Sextant) Cancer (Crab) Draco (Dragon) Musca (Fly) Taurus (Bull) Canis Venatici (Hunting Dogs) Equuleus (Little Horse) Norma (Carpenter’s Square) Telescopium (Telescope) Canis Major (Big Dog) Eridanus (River) Octans (Octant) Triangulum (Triangle) Canis Minor (Little Dog) Fornax (Furnace) Ophiuchus (Serpent Bearer) Triangulum Australe (Southern Triangle) Capricornus (Sea Goat) Gemini (Twins) Orion (Hunter) Tucana (Toucan) Carina (Keel) Grus (Crane) Pavo (Peacock) Ursa Major (Big Bear) Cassiopeia (Queen) Hercules (Hercules) Pegasus (Winged Horse) Ursa Minor (Little Bear) Centarus (Centaur) Horologium (Clock) Perseus (Hero) Vela (Sails) Cepheus (King) Hydra (Water Serpent) Phoenix (Phoenix) Virgo (Maiden) Cetus (Whale) Hydrus (Water Snake) Pictor (Painter’s Easel) Volans (Flying Fish) Chamaeleon (Chameleon) Indus (Indian) Pisces (Fishes) Vulpecula (Fox) 5 Diurnal Motion • Look at the diagram and imagine yourself standing somewhere on Earth. The Earth is spinning. What would you see over the course of a day when you look up at the celestial sphere??? 6 Diurnal Motion • The Earth’s axis of rotation also defines the key places on the celestial sphere. • The celestial North pole is over the Earth’s north pole. • The celestial equator is the extension of the Earth’s equator. • The meridian divides east from west on the sky. 7 Diurnal Motion • Each day, the Earth rotates once (west-to-east) on its axis. This causes us to face different directions and see different stars. The stars’ daily (diurnal) motion reflects the Earth’s spin. 8 Risings and Settings The spin of the Earth causes the stars to appear to rotate about the celestial pole. Some stars are circumpolar and never set, while others dip below the horizon. Which stars are which depends on where you are on Earth. The 2nd magnitude star Polaris happens to be very near the 9 North celestial pole. The Yearly Motion In addition to rotating, the Earth also revolves about the Sun. As the Earth revolves the Sun is projected in front of different constellations at different times of year. The path the Sun takes across heavens is called the ecliptic. The constellations which the Sun passes through are zodiac constellations. Because the Sun is bright, we can only see some constellations at certain times of year. 10 The Seasons Since the plane of the ecliptic is tilted 23.5° with respect to the celestial equator, we have seasons. 11 The Motion of the Sun From the Geocentric point of view, the Sun moves from the northern part to the southern part of the sky with the seasons. When the Sun is furthest north (south), it’s the summer (winter) solstice. When the Sun crosses the celestial equator, it’s the 12 vernal (or autumnal) equinox. The Day Because the Earth revolves about the Sun, the word “day” has two definitions. A sidereal day is one rotation with respect to the stars. A solar day is one rotation with respect to the Sun. Sidereal and solar days differ by about 4 minutes. 13 Precession In addition to its rotation and revolution, the Earth’s axis also precesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about 26,000 years. 14 Because of precession: • Polaris won’t always be the North Star • A sidereal year is not quite a Julian year. • Orion won’t always be a winter constellation • Horoscopes are all wrong (since those tables don’t include precession.) 15 The Lunar Period The Moon moves westto-east in the sky (like the Sun), and takes about a month to circle the Earth (hence the word month). But once again, there’s a difference between the Moon’s sidereal period with respect to the stars (27 days), and the synodic period with respect to the Sun (29 days). 16 Phases of the Moon The Moon reflects light from the Sun, so its phase tells you the relative position of the Sun. You can therefore tell time from the Moon! 17 Phases of the Moon We tell time by the Sun: it rises around 6 a.m., is high in the sky at noon, and sets around 6 p.m. The phase of the Moon tells you the angle between the Moon and Sun. So when you see the Moon, you know where the Sun is! 18 Phases of the Moon Full Moon occurs when the Moon is opposite in the sky from the Sun. This is called opposition. New Moon occurs when the Moon is in the same direction as the Sun, i.e., in conjunction. 19 The Moon’s Shadow The Moon’s orbital plane is tilted by 5.2 degrees from the ecliptic plane. Hence, ½ the time, the Moon is slightly north of the ecliptic (and ½ the time, it is south of the ecliptic). The shadow of one body very rarely falls on the other. 20 Lunar Eclipses If the Moon crosses the ecliptic plane while exactly opposite the Sun, it will fall in the Earth’s shadow. This is a lunar eclipse. This happens 21 roughly twice a year. Solar Eclipses If the Moon crosses the ecliptic plane while exactly in front of the Sun, a tiny piece of the Earth will fall in the Moon’s shadow. This is a solar eclipse. Since the Moon and Sun appear to be the same size in the sky, the alignment of a total solar eclipse must be perfect. 22 Total Solar Eclipse You only get to see this outer atmosphere (corona) of the Sun if 23 the Sun’s entire body is blocked out. 24 Partial Solar Eclipse If you’re on the edge of an eclipse path and only a slice of the Sun is blocked out, it’s called a partial eclipse. These are not very exciting, since the uneclipsed part of the Sun is still extremely bright. 25 Annular Eclipses Because the Moon’s orbit about the Earth is not perfectly circular, it is sometimes too far away to block out the whole Sun, even when perfectly aligned. When this happens, you get an annular eclipse. It is rarer than a total solar eclipse, but, like other partial eclipses, they are, in general, pretty boring. 26 Solar Eclipse Paths through 2025 27