astro 101 principles of astronomy. instructor: jerome a. orosz (rhymes with “boris”) contact:...

ASTRO 101Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”)Contact:• Telephone: 594-7118• E-mail: jorosz@mail.sdsu.edu• WWW:

http://mintaka.sdsu.edu/faculty/orosz/web/• Office: Physics 241, hours T TH 2:00-3:20

Text:

“Discovering the Essential Universe, Fifth Edition”

by Neil F. Comins

Course WWW Pagehttp://mintaka.sdsu.edu/faculty/orosz/web/ast101_spring2014.htmlNote the underline: … ast101_spring2014.html …

Also check out Nick Strobel’s Astronomy Notes:http://www.astronomynotes.com/

• Homework due February 4: 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 February 4.

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.http://www.astronomynotes.com/nakedeye/s4.htm

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=90o at the north celestial pole, 0o at the equator, and -90o at the south celestial pole.

– http://www.astronomynotes.com/nakedeye/s6.htm

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?

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 (http://www.astronomynotes.com/))

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?

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:http://www.worldtimezone.com/datetime.htm

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?

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