ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

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

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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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/

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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

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Homework

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

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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.
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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)

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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

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Question for Today

• Why dont we have eclipses every month?

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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???

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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.

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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.

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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.

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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!

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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.

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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/))
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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.

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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.

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Seasons in Detail

• If we do some careful observations, we find

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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.

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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.

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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).

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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

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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.

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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.

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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.

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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.

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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.

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In Detail

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

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What Causes the Seasons?
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What Causes the Seasons?

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

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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).

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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.

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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.

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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.

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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.

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What Causes the Seasons?

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

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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.

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What Causes the Seasons?

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

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What Causes the Seasons?

• Here is a slide from NASA and NOAA.

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What Causes the Seasons?

• A slide from Nick Strobel.

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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.

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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.

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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/).

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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.

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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.

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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.

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Next

• The Moon

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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.

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How Long is one Month?
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How Long is one Month?

• It depends

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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.

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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.

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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.

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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.

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What Causes the Phases of the Moon?
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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.

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What Causes the Phases of the Moon?

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

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What Causes the Phases of the Moon?

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

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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.

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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/)
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The Seven Day Week?
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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.
• The Moon.

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.
• The Moon.
• Mars.

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.
• The Moon.
• Mars.
• Mercury.

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.
• The Moon.
• Mars.
• Mercury.
• Jupiter.

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun.
• The Moon.
• Mars.
• Mercury.
• Jupiter.
• Venus.

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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.

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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.

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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.

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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

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The Seven Day Week?

• There are seven bright objects in the sky that
  are not stars
• The Sun. Sunday
• The Moon. Monday
• Mars. Tuis Day (Norse)
• Mercury. Wodens Day (Norse)
• Jupiter. Thors Day (Norse)
• Venus. Freya Day (Norse)
• Saturn. Saturday

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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

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NextLunar and Solar Eclipses