Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: 594-7118 E-mail: orosz@sciences.sdsu.edu 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 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/

Astronomy Help Room No appointment needed! Just drop by! Where: Room 215, physics-astronomy building. When: Monday: 12-2, 4-6 PM Tuesday: 12-1 PM; 4-6 PM Wednesday: 12-2, 5-6 PM Thursday: 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: 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. 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: The Sun.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. The Moon.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. The Moon. Mars.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. The Moon. Mars. Mercury.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. The Moon. Mars. Mercury. Jupiter.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. The Moon. Mars. Mercury. Jupiter. Venus.

The Seven Day Week? There are seven bright objects in the sky that are not stars: 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: The Sun. Sunday The Moon. Mars. Mercury. Jupiter. Venus. Saturn.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. Sunday The Moon. Monday Mars. Mercury. Jupiter. Venus. Saturn.

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. Sunday The Moon. Monday Mars. Mercury. Jupiter. Venus. Saturn. Saturday

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. Sunday The Moon. Monday Mars. Tui’s Day (Norse) Mercury. Woden’s Day (Norse) Jupiter. Thor’s Day (Norse) Venus. Freya’ Day (Norse) Saturn. Saturday

The Seven Day Week? There are seven bright objects in the sky that are not stars: The Sun. Sunday The Moon. Monday Mars. Martes in Spanish Mercury. Miercoles in Spanish Jupiter. Jueves in Spanish Venus. Viernes in Spanish Saturn. Saturday

Next: Lunar and Solar Eclipses