ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 3:30-5:00

Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins

Course WWW Page Note the underline: … ast101_fall2012.html … Also check out Nick Strobel’s Astronomy Notes:

Where: Room 215, physics-astronomy building. No appointment needed! Just drop by! 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 is the shape of the Earth, and is it stationary? We all know the answers to the above, but can you give evidence for the correct conclusion without using modern technology? –The stars change as you go north-south –The shape of the Earth’s shadow during lunar eclipse –The manner in which ships disappear.

Questions for Today What causes the seasons? What causes the phases of the moon Why don’t we see eclipses every month?

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

The Celestial Sphere Imagine the sky as a hollow sphere with the stars attached to it. This sphere rotates once every 24 hours. This imaginary sphere is called the celestial sphere. Even though we know it is not the case, it is useful to imagine the Earth as being stationary while the celestial sphere rotates around it.

The Celestial Sphere The north celestial pole is directly above the north pole on the Earth. The south celestial pole is directly above the south pole on the Earth. The celestial equator is an extension of the Earth’s equator on the sky. The zenith is the point directly over your head. The horizon is the circle 90 degrees from the zenith.

The Celestial Sphere The celestial poles and the celestial equator are the same for everyone. The zenith and the horizon depend on where you stand.

Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Altitude/Azimuth: –The altitude describes how many degrees the star is above the horizon, the azimuth describes how far the star is in the east-west direction from north. –The altitude and azimuth of a star is constantly changing owing to the motion of the star on the sky!

Stellar Coordinates and Precession There are a few ways to specify the location of a star (or planet) on the sky: Equatorial system: –Lines of longitude on the earth become right ascension, measured in units of time. The RA increases in the easterly direction. –Lines on latitude on the earth become declination, measured in units of degrees. DEC=90 o at the north celestial pole, 0 o at the equator, and -90 o at the south celestial pole. –

Stellar Coordinates and Precession The north celestial pole moves with respect to the stars very slowly with time, taking 26,000 years to complete one full circle.

The Clockwork of the Universe There are many familiar astronomical cycles:  The Day/Night cycle.  The phases of the Moon (the lunar cycle).  The seasons of the year.  The seven day week???

The Day/Night Cycle Every day, the Sun rises in the east, and sets in the west. At night, the stars move in fixed patterns. The specific paths depend on where you look…

The Day/Night Cycle The stars rotate about a fixed point in the northern sky (for observers in the northern hemisphere).

Day/Night Cycle We have the Sun rising and setting, and the stars moving in fixed patterns about a point in the sky. Two possible reasons: 1)The Earth is fixed in space, and the Sun and the “celestial sphere” move around it. 2)The Earth is spinning about its axis.

Day/Night Cycle The Earth is spinning on its axis, while it and the other planets revolve around the Sun. The apparent motion of the Sun defines the day. The rotation of the Earth affects weather patterns, ocean currents, the paths of missiles etc.

How Long is a Day?

It depends:

How Long is a Day? It depends:  If you measure the length of time between successive “noons” (the time when the Sun is at its highest point in the sky), the day is on average 24 hours. This is a “mean solar day.”

How Long is a Day? It depends:  If you measure the length of time between successive “noons” (the time when the Sun is at its highest point in the sky), the day is on average 24 hours. This is a “mean solar day.”  If you do the same thing, but with a star rather than with the Sun, the day is about 23 hours and 56 minutes. This is a “sidereal day”.

Solar vs. Sidereal Since the Earth is moving around the Sun, it has to rotate an extra bit each day so that the Sun appears in the same point in the sky. Image from Nick Strobel’s Astronomy notes (

Solar vs. Sidereal 1-6_SideralTime.html Your favorite star rises 4 minutes earlier each successive night. Eventually, it will rise when the Sun is still out. Hence, with a few exceptions, a given constellation is visible at night only during certain times of the year.

How Fast does the Earth Rotate?

1)Pretty fast: the circumference of the Earth is about 24,000 miles at the equator, so a point on the equator moves 1000 miles per hour (24,000 miles in 24 hours).

How Fast does the Earth Rotate? 1)Pretty fast: the circumference of the Earth is about 24,000 miles at the equator, so a point on the equator moves 1000 miles per hour (24,000 miles in 24 hours). 2)Pretty slow: just watch the hour hand on the clock for a while. That hand moves twice as fast as the earth!

What Time is it? A nice map of time zones:

The Cycle of the Sun The Sun would be in different constellations during certain times of the year, if you could see the stars in the day. Where the Sun is depends on the season.

The Cycle of the Sun The Sun would be in different constellations during certain times of the year, if you could see the stars in the day. Where the Sun is depends on the season.

Next: The seasons

Seasons in Detail: If we do some careful observations, we find:

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.

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  Getting warmer: spring  Warm weather: summer  Cooling off: fall

In Detail: In most places on Earth, the weather patterns go through distinct cycles:  Cold weather: winter  Getting warmer: spring  Warm weather: summer  Cooling off: fall These “seasons” are associated with the changing day/night lengths.

In Detail: In most places on Earth, the weather patterns go through distinct cycles:  Cold weather: winter, shorter daytime  Getting warmer: spring  Warm weather: summer  Cooling off: fall These “seasons” are associated with the changing day/night lengths.

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  Cooling off: fall These “seasons” are associated with the changing day/night lengths.

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 These “seasons” are associated with the changing day/night lengths.

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?