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ASTRO 101 Principles of Astronomy Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: • Telephone: 594-7118 • E-mail: [email protected] • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00 Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins Course WWW Page http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html Note the underline: … ast101_fall2012.html … Also check out Nick Strobel’s Astronomy Notes: http://www.astronomynotes.com/ No appointment needed! Just drop by! Where: Room 215, physics-astronomy building. When: • Monday: • Tuesday: • Wednesday: • Thursday: 12-2, 4-6 PM 12-1 PM; 4-6 PM 12-2, 5-6 PM 4-6 PM • Homework due September 6: Question 15 from Chapter 1 (Why is it warmer in the summer than in winter?) • Write down the answer on a sheet of paper and hand it in before the end of class on September 6. Questions from Before • What causes the seasons? The tilt of the Earth’s axis relative to its orbital plane gives rise to a changing pattern of solar heating… • What causes the phases of the moon? The relative position of the Sun, Moon, and Earth… • Why don’t we see eclipses every month? Coming up… Questions for Today • Why are the seasons opposite between the northern and southern hemispheres? • Why don’t we see eclipses every month? • What is gravity? Next: Discovering the Night Sky Coming Up: • Introduction to the Sky – – – – Constellations Stellar Brightness Stellar coordinates and the Celestial Sphere The “clockwork” of the sky • Day/night • Phases of the moon • The seasons Next: • The seasons In Detail: • If we do some careful observations, we find: 1) The length of the daylight hours at a given spot varies throughout the year: the Sun is out a longer time when it is warmer (i.e. summer), and out a shorter time when it is colder. 2) On a given day, the length of the daylight hours depends on where you are on Earth, in particular it depends on your latitude: e.g. in the summer, the Sun is out longer and longer the further north you go. In Detail: • Near the North Pole, the Sun never sets in the middle of the summer (late June). • Likewise, the Sun never rises in the middle of the winter (late December). In Detail: • In most places on Earth, the weather patterns go through distinct cycles: Cold weather: winter, shorter daytime Getting warmer: spring, equal daytime/nighttime Warm weather: summer, longer daytime Cooling off: fall, equal daytime/nighttime • These “seasons” are associated with the changing day/night lengths. In Detail: • When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and the other way around. What Causes the Seasons? • Is the Earth closer to the Sun during summer, and further away during winter? (This was the most commonly given answer during a poll taken at a recent Harvard graduation). • No! Otherwise the seasons would not be opposite in the northern and southern hemispheres. What Causes the Seasons? • The Earth moves around the Sun. A year is defined as the time it takes to do this, about 365.25 solar days. • This motion takes place in a plane in space, called the ecliptic. • The axis of the Earth’s rotation is inclined from this plane by about 23.5 degrees from the normal. What Causes the Seasons? • The axis of the Earth’s rotation points to the same point in space (roughly the location of the North Star). • The result is the illumination pattern of the Sun changes throughout the year. What Causes the Seasons? • Here is an edge-on view, from the plane of the Earth’s orbit. What Causes the Seasons? • Here is a view from slightly above the Earth’s orbital plane. What Causes the Seasons? • A slide from Nick Strobel. What Causes the Seasons? • Because of the tilt of the Earth’s axis, the altitude the Sun reaches changes during the year: It gets higher above the horizon during the summer than it does during the winter. What Causes the Seasons? • Because of the tilt of the Earth’s axis, the altitude the Sun reaches changes during the year: It gets higher above the horizon during the summer than it does during the winter. • Also, the length of the daytime hours changes during the year: the daylight hours are longer in the summer and shorter in winter. What Causes the Seasons? • The altitude of the Sun matters: when the Sun is near the horizon, it does not heat as efficiently as it does when it is high above the horizon. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/). What Causes the Seasons? • The Sun’s daily path across the sky depends on the time of year… What Causes the Seasons? • Winter: The combination of a short daytime and a Sun that is relatively low above the horizon leads to much less heating in the day, plus a longer period of cooling at night. Overall, it is colder. What Causes the Seasons? • Summer: The combination of a long daytime and a Sun that is relatively high above the horizon leads to much more heating in the day, plus a shorter period of cooling at night. Overall, it is warmer. What Causes the Seasons? • Spring and Fall: The number of hour of daylight is about equal to the number of nighttime hours, leading to roughly equal times of heating and cooling. Next: The Moon The Phases of the Moon • Next to the Sun, the Moon is the most noticeable object in the sky. • The lunar cycle is the basis of the month. How Long is one Month? How Long is one Month? • It depends: How Long is one Month? • It depends: If you use the Sun as a reference, the Moon takes 29.5 days to complete one orbit around the Earth. How Long is one Month? • It depends: If you use the Sun as a reference, the Moon takes 29.5 days to complete one orbit around the Earth. If you use a star as a reference, the moon takes 27.3 days to go around the Earth. How long is one Month? • During the course of 27 days, the Earth has moved around a substantial part of its orbit about the Sun. • It takes an extra 2 days for the Moon to “catch up” with the Sun. How Many Months are in a Year? • It depends: – 365.25/29.5=12.4 if you use the Sun as the reference. – 365.25/27.3=13.4 if you use a star as the reference. – 12 calendar months, with each calendar month being slightly longer than one lunar cycle. What Causes the Phases of the Moon? What Causes the Phases of the Moon? • The full Moon always rises just after sunset. • The crescent Moon always points towards the Sun. • A crescent Moon sets shortly after sunset, or rises just before sunrise. • The Moon is illuminated by reflected sunlight. What Causes the Phases of the Moon? • The full Moon always rises just after sunset. • A crescent Moon sets shortly after sunset. What Causes the Phases of the Moon? • The full Moon always rises just after sunset. • A crescent Moon sets shortly after sunset. What Causes the Phases of the Moon? • The lit side of the Moon always faces the Sun. • Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month. What Causes the Phases of the Moon? • The lit side of the Moon always faces the Sun. • Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month. The Seven Day Week? The Seven Day Week? • There are seven bright objects in the sky that are not stars: The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) The Sun. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) The Sun. 2) The Moon. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) The Sun. 2) The Moon. 3) Mars. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) The Sun. The Moon. Mars. Mercury. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) The Sun. The Moon. Mars. Mercury. Jupiter. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. Sunday The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. Sunday Monday The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. Sunday Monday Saturday The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. Sunday Monday Tui’s Day (Norse) Woden’s Day (Norse) Thor’s Day (Norse) Freya’ Day (Norse) Saturday The Seven Day Week? • There are seven bright objects in the sky that are not stars: 1) 2) 3) 4) 5) 6) 7) The Sun. The Moon. Mars. Mercury. Jupiter. Venus. Saturn. Sunday Monday Martes in Spanish Miercoles in Spanish Jueves in Spanish Viernes in Spanish Saturday Next: Lunar and Solar Eclipses