Astronomy

1
Astronomy
Background from Chapter 1

• Scientific Method
• Measurement

2
How Do We Know All This?
Scientific Method Allows theories to evolve
through a combination of observations,
theoretical reasoning, and predictionwhich in
turn suggests new observations.
3
How Do We Know All This?
Testable ? experimental verification Tested ?
continual testing allows theories to be
changed Simple ? Occams razor Elegant ? why
make a new theory when you can amend an
old one?
4
Chain of Scientific Method
Observation
Theory
Prediction
5
Scientific Method
Gravity is a theory? Yes, however, this theory
has guided scientists calculations with
technology from the days of Sir Isaac Newton to
Space Travel calculations
6
Observations
The natural thing to do with observations, for
humans, is to give them an order. Long ago,
people have recognized patterns in the sky and
applied mythology . . .
7
Constellation
constellation A human grouping of stars in the
night sky into a recognizable pattern
8
Constellations
9
Constellations
Constellations Near Orion The region of the sky
near Orion, together with some neighboring
constellations. Some prominent stars are labeled
in lowercase letters. The 88 constellations span
the entire sky, so that every astronomical object
lies in precisely one of them
10
Constellations
Benefits to mapping the night sky?
11
Constellations
Benefits to mapping the night sky?
Directional/Navigational Culture/Myths Seasons Ast
rology (i.e. Miss Cleo)
12
Constellations
Now remember that space is 3-D
1,000 light-years
13
celestial sphere
Surrounding earth we see a canopy of stars
resembling an astronomical painting on a heavenly
ceiling.
14
Measurement
light year The distance that light, moving at a
constant speed of 300,000 km/s, travels in one
year. One light year is about 10 trillion
kilometers
15
Angular Measurement -- 1

• A full circle contains 360 degrees (360).
• Thus, the half-circle that stretches from horizon
  to horizon, passing directly overhead and
  spanning the portion of the sky visible to one
  person at any one time, contains 180.

16
Angular Measurement -- 2

• Each 1 increment can be further subdivided into
  fractions of a degree, called arc minutes.
• There are 60 arc minutes (written 60) in one
  degree.
• Both the Sun and the Moon project an angular size
  of 30 arc minutes on the sky. Your little finger,
  held at arms length, does about the same,
  covering about a 40' slice of the 180
  horizon-to-horizon arc.

17
Angular Measurement -- 3

• An arc minute can be divided into 60 arc seconds
  (60).
• Put another way, an arc minute is 1/60 of a
  degree, and an arc second is 1/60 x 1/60 1/3600
  of a degree.
• An arc second is an extremely small unit of
  angular measureit is the angular size of a
  centimeter-sized object (a dime, say) at a
  distance of about two kilometers
• (a little over a mile).

18
Angular Measurement
19
Angular Measurement -- 4

• The angular size of an astronomical object
  depends on
• its actual size
• 2. its distance from us

20
Angular Measurement -- 5

• For example, the Moon, at its present distance
  from Earth, has an angular diameter of 0.5, or
  30.
• If the Moon were twice as far away, it would
  appear half as big15 acrosseven though its
  actual size would be the same.
• Thus, angular size by itself is not enough to
  determine the actual diameter of an objectthe
  distance must also be known.

21
Solar Days

• 24-hour solar day, is our basic social time unit.
  The daily progress of the Sun and the other stars
  across the sky is known as diurnal motion.
• As we have just seen, it is a consequence of
  Earths rotation.
• But the stars positions in the sky do not repeat
  themselves exactly from one night to the next.
• Each night, the whole celestial sphere appears to
  be shifted a little relative to the horizon,
  compared with the night before

22
Sidereal Days

• Because of this shift, a day measured by the
  starscalled a sidereal day after the Latin word
  sidus, meaning "star"differs in length from a
  solar day.
• Evidently, there is more to the apparent motion
  of the heavens than simple rotation.

23
Sidereal Days

• sidereal day
• The time needed for a star on the celestial
  sphere to make one complete rotation in the sky.

24
Night Sky

• Typical Night Sky (a) A typical summer sky above
  the United States. Some prominent stars (labeled
  in lowercase letters) and constellations (labeled
  in all capital letters) are shown. (b) A typical
  winter sky above the United States.

25
Night Sky

• The 12 constellations through which the Sun
  passes as it moves along the eclipticthat is,
  the constellations we would see looking in the
  direction of the Sun, if they werent overwhelmed
  by the Suns lighthad special significance for
  astrologers of old. These constellations are
  collectively known as the zodiac.

26
Night Sky

• The Zodiac The view of the night sky changes as
  Earth moves in its orbit about the Sun. As drawn
  here, the night side of Earth faces a different
  set of constellations at different times of the
  year. The twelve constellations named here
  comprise the astrological zodiac.