ASTRO 101

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ASTRO 101

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Title: ASTRO 101

1
ASTRO 101

Principles of Astronomy

2
Instructor Jerome A. Orosz
(rhymes with boris)Contact

Telephone 594-7118
E-mail orosz_at_sciences.sdsu.edu
WWW http//mintaka.sdsu.edu/faculty/orosz/web/
Office Physics 241, hours T TH 330-500

3
Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
4
Course WWW Page

http//mintaka.sdsu.edu/faculty/orosz/web/ast101_f
all2009a.html
Note the underline ast101_fall2009a.html
Also check out Nick Strobels Astronomy Notes
http//www.astronomynotes.com/

5
Astronomy Help Room Hours

Monday 1200-1300, 1700-1800
Tuesday 1700-1800
Wednesday 1200-1400, 1700-1800
Thursday 1400-1800, 1700-1800
Friday 900-1000, 1200-1400
Help room is located in PA 215

6
Homework

Assigned question due September 10 Question 13,
Chapter 2 (Why are the seasons reversed in the
southern hemisphere relative to the northern
hemisphere?)

7
Homework

Assigned question due September 17 Question 9,
Chapter 3 (Review Galileos telescope discoveries
and explain why they supported the Copernican
model and contradicted the Ptolemaic model.)
OR
Go to a planetarium show in PA 209
Thu. Sep. 10 1000-1100, 1600-1700
Fri. Sep. 11 1400-1500
Mon. Sep. 14 1000-1100, 1500-1600
Tue. Sep. 15 1100-1200
Wed. Sep. 16 1100-1200
Thu. Sep. 17 1400-1500
Fri. Sep. 18 1600-1700

Sign up for a session outside PA 209 Hand in a
sheet of paper with your name and the date and
time of the session.
8
Coming Up

Introduction to the Sky
Constellations
Stellar Brightness
Stellar coordinates
The clockwork of the sky
Day/night
Phases of the moon
The seasons
Solar and Lunar Eclipses
A Brief History of Astronomy (Chapter 3)

9
Questions from Before

What causes the seasons?
The Earths rotation axis is tilted 23.5 degrees
with respect to the normal of its orbital plane.
This gives rise to a varying illumination pattern
by the Sun

10
Question for Today

Why dont we have eclipses every month?

11
The Clockwork of the Universe

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

12
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
The Earth is fixed in space, and the Sun and the
celestial sphere move around it.
The Earth is spinning about its axis.

13
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.

14
Day/Night Cycle

In what direction does the Earth rotate (as seen
from above the North Pole)?
Clockwise
Counterclockwise, since the Sun rises in the east
and sets in the west.

15
How Fast does the Earth Rotate?

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).
Pretty slow just watch the hour hand on the
clock for a while. That hand moves twice as fast
as the earth!

16
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.

17
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 Strobels Astronomy notes
(http//www.astronomynotes.com/))
18
Solar vs. Sidereal

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.

19
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.

20
Seasons in Detail

If we do some careful observations, we find

21
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.

22
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.

23
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).

24
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

25
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.

26
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.

27
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.

28
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.

29
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.

30
In Detail

When it is summer in the northern hemisphere, it
is winter in the southern hemisphere, and the
other way around.

31
What Causes the Seasons?
32
What Causes the Seasons?

Is the Earth closer to the Sun during summer,
and further away during winter?

33
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).

34
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.

35
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.

36
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.

37
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 Earths rotation is inclined from
this plane by about 23.5 degrees from the normal.

38
What Causes the Seasons?

The axis of the Earths rotation points to the
same point in space (roughly the location of the
North Star).

39
What Causes the Seasons?

The axis of the Earths 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.

40
What Causes the Seasons?

Here is an edge-on view, from the plane of the
Earths orbit.

41
What Causes the Seasons?

Here is a slide from NASA and NOAA.

42
What Causes the Seasons?

A slide from Nick Strobel.

43
What Causes the Seasons?

Because of the tilt of the Earths 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.

44
What Causes the Seasons?

Because of the tilt of the Earths 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.

45
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 Strobels Astronomy Notes
(http//www.astronomynotes.com/).

46
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.

47
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.

48
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.

49
Next

The Moon

50
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.

51
How Long is one Month?
52
How Long is one Month?

It depends

53
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.

54
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.

55
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.

56
How Many Months are in a Year?

It depends
365.25/29.512.4 if you use the Sun as the
reference.
365.25/27.313.4 if you use a star as the
reference.
12 calendar months, with each calendar month
being slightly longer than one lunar cycle.

57
What Causes the Phases of the Moon?
58
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.

59
What Causes the Phases of the Moon?

The full Moon always rises just after sunset.
A crescent Moon sets shortly after sunset.

60
What Causes the Phases of the Moon?

The full Moon always rises just after sunset.
A crescent Moon sets shortly after sunset.

61
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.

62
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.

Image from Nick Strobel (http//www.astronomynotes
.com/)
63
The Seven Day Week?
64
The Seven Day Week?