Astr 1050 Wed. Jan. 28, 2015 • Begin Chapter 2 topics – The Celestial Sphere – Moon Phases – Eclipses – Planetary Motion Reading: For Today: Begin Chapter 2 For Friday: Finish Chapter 3 Ch. 2 Homework on Mastering Astronomy (Due Next Monday) Chapter 2: The View From The Earth • Imagine being in a rotating restaurant on top of a tall building. All the outside objects are very far away – much farther than the distance across the room. • Paint the view on the windows – and keep the people near the center of the room – away from the windows themselves. • Can the people tell if the room is rotating, or if the painted windows are just moving around the room? • Which is more reasonable – a rotating room or rotating painted windows? The Canada National Tower in Toronto Celestial Sphere Nomenclature • HORIZON: The horizontal circle which separates the part of the sky visible to you and the part of the sky hidden by the earth. • ZENITH: The point on the sky directly overhead. • MERIDIAN: The circle which starts on the northern horizon, runs through the zenith, continuing on to the southern horizon. It separates the eastern half of the sky from the western half. • CELESTIAL POLES: The points where the extension of the rotation axis of the earth would intersect the celestial sphere. • CELESTIAL EQUATOR: The circle around the sky which would be a projection of the earth’s equator. Limiting Cases From Voyages through the Universe, by Fraknoi et al. – At the Earth’s north pole, looking overhead all stars appear to circle around the north celestial pole. – At the equator: • Stars on the celestial equator rise in the east, move overhead, then set in the west • The N and S celestial poles are just on your N and S horizons, and stars near those points still circle around them. But those stars are only visible for the upper half of their circles. Intermediate cases like Laramie From Voyages through the Universe, by Fraknoi et al. – Stars close enough to the north celestial pole are always above the horizon, and just circle the pole star. (CIRCUMPOLAR STARS) – Stars on the celestial equator rise in the east, move higher along a slanted path which crosses the “meridian” to the south of the zenith, then descend and set due west. – Stars far enough to the south never make it above the horizon. Star Motion from the Northern Hemisphere (Laramie, for instance) Precession of the Earth • The earth’s axis of rotation is tilted 23.50 relative to the plane containing the sun and other planets. • The gravity from the Sun and moon is trying to tip the earth just like gravity is trying to tip a spinning top. • As with the top, the axis of the earth wobbles or PRECESSES in space, with a 26,000 year period. • Because the directions to the celestial poles are defined by the spin axis – those poles move with time. – It isn’t that the stars move – it is that the grid we paint on the celestial sphere has to be redrawn from time-to-time. – Eventually Polaris will not be the “pole” star. (From Horizons, by Seeds) Chapter 2: Cycles in the Sky Motion of the Sun through the year – Plot position of Sun relative to stars, over one full year. • Complicated by fact you can’t see Sun and stars at same time. • Once you have full map of sky, you can determine the time of year by seeing what stars are opposite sun 12 hours later. (From Horizons, by Seeds) • Path of sun around the celestial sphere is called the ECLIPTIC • Set of constellations through which it passes is called the ZODIAC Plotting the Ecliptic on the Celestial Sphere • The ecliptic is tilted relative to the celestial equator by 23.5o • The sun is at the northernmost point on the ecliptic on June 22 – a time and location called the SUMMER SOLSTICE • The Sun is at the southernmost point on the ecliptic on Dec. 22 – a time and location called the WINTER SOLSTICE • The Sun just crossing the equator going N on March 21 – a time and location called the VERNAL EQUINOX (From Horizons, by Seeds) The Sun is just crossing the equator going S on Sept. 22 – a time and location called the AUTUMNAL EQUINOX Consider the Sun’s daily motion thru the year Key point: Consider sun fixed at a given spot on ecliptic over the period of one day. • At the Vernal Equinox Sun is on the celestial equator. • At the Autumnal Equinox the Sun is also on the celestial equator. • It rises due E, sets due W • It is up exactly 12 hours • At the Summer solstice Sun is a “northern” star. • It rises N of E, sets N of W • It is up more than 12 hours • At the Winter Solstice it is a “southern” star • It rises S of E, sets S of W • It is up less than 12 hours (From our Text: Horizons, by Seeds) How the Sun’s location affects the seasons: • The angle of the sun’s rays: • In the summer it passes closer to overhead and therefore shines more directly on the summer hemisphere • The time the Sun is up • In the summer it spends more than 12 hours above the horizon. • The seasons are NOT due to the slightly elliptical shape of the Earth’s orbit and the fact that it is slightly closer to the Sun during part of the year. • Test of that hypothesis: If the distance were the cause, then when it was summer in the northern hemisphere, what season would it be in the southern hemisphere? SUN (From our Text: Horizons, by Seeds) Special Locations on the Earth • How close to the North Pole do we need to go before the Summer Solstice sun becomes a “circumpolar star” and is above the horizon all day? • Within 23.5o of the pole: THE ARCTIC CIRCLE • How close to the equator do we need to get before the Summer Solstice sun passes directly overhead rather than somewhat to the south: • Within 23.5o of the equator: The TROPICS (From our Text: Horizons, by Seeds) Why are the planets found near the ecliptic? • The ecliptic is defined by the plane of the Earth’s orbit • If the other planets are always found near the ecliptic, they must always be located near the plane of the Earth’s orbit – at most slightly above or below it. • The planes of their orbits around the sun must almost match the Earth’s • Their slight motions above and below the ecliptic means the match isn’t exact. (Their orbits are slightly tilted relative to ours.) From our text: Horizons, by Seeds Superior vs. Inferior Planets • Superior planets (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) have orbits larger than the earth and can appear opposite the sun in the sky. They can be up at midnight. Never show phases. • Inferior planets (Mercury, Venus) have orbits smaller than earth and can never appear far from the Sun. They form “morning stars” or “evening stars” visible a little before sunrise or after sunset. Show phases. From our text: Horizons, by Seeds Inferior Planets • Inferior planets (Mercury, Venus) have orbits smaller than earth and can never appear far from the Sun. They form “morning stars” or “evening stars” visible a little before sunrise or after sunset. From our text: Horizons, by Seeds Effect of Elliptical Orbit on Climate • Seasons almost entirely due to TILT of Earth – Seasons opposite (not the same) in N & S Hemispheres • Earth’s orbit slightly elliptical – Slightly closer to the sun in N. Hemisphere Winter • But this changes as tilt precesses in 26,000 yr cycle – Expect N. Hemisphere winter to be slightly milder • Positions of continents and oceans actually more important • Effect is important for Mars -- more elliptical orbit • Cyclic variations in climate as tilt precesses and tilt and ellipticity also gets slightly larger and smaller • VERY IMPORTANT TOPIC (Re: Climate Change) For Friday: • The motion of the moon • We will cover phases of the moon, eclipses, and other really good stuff! (Should cover material needed for last few homework problems as well.) • Begin History of Astronomy: Reading: Chapter 3 of Cosmic Perspectives .