Download - AST 1.2 PPT
- 1. 1.2THE SKY
2. Constellations
A constellation is simply a grouping, or pattern, of stars.
In ancient times, constellations only referred to the brightest
starsappearing to form groups.
They were believed to represent great heroes and mythological
figures.
Today, they are well defined regions of the sky, regardless of the
presence of bright stars.
3. Constellations
There are 88 official constellations we study today.
In addition to those, the sky contains a number of less formally
defined groupings called asterisms.
Example 1: The Big Dipper is a well-known asterism of the
constellation, Ursa Major(The Great Bear).
4. Constellations
Example 2: The Great Square of Pegasus (includes stars from both
the Pegasus and Andromeda constellations).
5. Constellations
The stars of constellations only appear to be close together.This
is often known as the projection effect.
They may actually be located at very different distances from
us.
6. Constellations and Their Stars
7. 8. Naming Stars
In addition to naming groups of stars, ancient astronomers gave
individual names to the brightest individual stars.
They are named by a Greek letter (, , ) according to their relative
brightness within a given constellation + the possessive form of
the name of the constellation:
Example: brightest star in constellation Canis Majoris alpha
CanisMajoris (CanisMajoris).
Brightest star in a constellation is usuallyalpha (), the second
brightest beta (), and so on in descending order.
9. Naming Stars
In the constellation, Orion, however, that is not the case. Orionis
is actually brighter than Orionis.
Orionis=Betelgeuse
Orionis =Rigel
Betelgeuse
Alnitak
Alnilam
Mintaka
Rigel
10. Brightness of Stars
Astronomers measure the brightness of stars using what is known as
the magnitude scale.
Ancient astronomers divided this scale into 6 classes.
The brightest: 1st magnitude stars, those fainter were known as 2nd
magnitude stars, etc. in order of decreasing brightness down to 6th
magnitude (faintest visible to the unaided human eye).
Sometimes, stars are so bright, they extend into the negative
numbers.
Example: Sirius (brightest star in the sky) = -1.47
We also extend the faint end of the scale for stars extremely faint
to us, requiring us to use a telescope.
These numbers are known as apparent visual magnitude, (mv),
describing how stars look to human eyes observing from Earth.
The actual magnitude of a star from a standard distance is known as
the absolute visual magnitude, (Mv).
11. Brightness of Stars
12. Magnitude and Intensity
Flux is a measure of the light energy from a star hitting one
square meter in one second.
Defines the intensity of starlight.
If 2 stars have intensities IA and IB, the ratio of their
intensities is IA/IB.
Astronomers have defined the magnitude scale so two stars differing
by 5 magnitudes have an intensity ratio of 100.
Therefore, stars only differing by 1 magnitude must have an
intensity ratio of 5100, which equals 2.512.
The light of one star is 2.512 times more intense.
Intensity Ratio Formula
IA
IB
=2.512(m m)
A
B
13. Magnitude and Intensity
Examples:
Two stars differ by 6.32 magnitudes.What is their intensity
ratio?
Two stars differ by 4 magnitudes.What is their intensity
ratio?
14. Magnitude and Intensity
What if we know the intensity ratio and want to find the magnitude
difference?
Difference In Magnitude Formula
IA
IB
- Example:
15. Light from the Sun is 24.2 times more intense than light
from the North Star, Polaris.What is the magnitude difference
between the two stars.(
)
mB mA = 2.5 log
16. The Celestial Sphere
17. The Celestial Sphere
18. The Celestial Sphere
The distance between two stars on the celestial sphere can only be
given as the difference between the directions in which we see the
stars.
Therefore, they are measured as angles, either in:
degrees (o):
Full circle = 360o
arc minutes ():
1o = 60
arc seconds ():
1 = 60
19. The Celestial Sphere
From geographic latitude l (northern hemisphere), you see the north
celestial pole l degrees above the northern horizon.
From geographic location l (southern hemisphere), you see the south
celestial pole l degrees above the southern horizon.
Celestial equator culminates 90 - labove the horizon.
90o - l
l
20. The Celestial Sphere
21. Apparent Motion
Looking north from mid-northern latitudes, some stars appear to
circle around the north celestial pole.
22. Apparent Motion
Constellations that never rise or set are called circumpolar
constellations.
Below are the 5 we see from our latitude:
Ursa Major
Ursa Minor
Cepheus
Cassiopeia
Draco
23. The Sun and Its Motions
Earth has 3 distinct motions:
Rotation the turning, or spinning, of a body on its axis.
Revolution the motion of a body, such as a planet or moon, along
its orbit around some point in space.
Precession the wobbling of a body around its axis of
rotation.
24. Rotation
The main results of Earths rotation (W to E) is day and
night.Rotating @ 1000 mph.
Axis of rotation tilted about 23.5
25. Synodic vs. Sidereal Day
We can measure Earths day in 2 ways:
Synodic (Solar) one complete rotation with respect to the Sun: 24
hours
Sidereal one complete rotation with respect to distant stars: 23
hours, 56 minutes
26. Revolution
Earths revolves around the Sun in a very slight elliptical
orbit.Revolving at an average speed of 67,000 mph.
Average distance of 93 million miles.
Due to its slightly elliptical orbit, the distance will vary:
Perihelion Earth is closest to the Sun (91 million miles in
January).
Aphelion: Earth is farthest from the Sun (95 million miles in
June).
27. Annual Motion of the Sun
During the day, the Sun appears to move across the sky once it
rises in the East until it eventually sets in the West.
Over the course of a year, the Sun appears to move East against the
background of the stars.
The apparent path of the Sun against the background of the stars is
called the ecliptic.
If the sky were a great screen, the ecliptic would be the shadow
cast by Earths orbit.
Stars are present during the day (not visible due to Sun).
Certain constellations appear in the ecliptic path at different
times of the year (sun is blocking them) and are known as the
zodiac constellations.
Recently, astronomers have pointed out the original zodiac
constellations may be outdated since they were created 3000 years
ago and the Earth has moved slightly due to precession.
Believed that the Sun moves into a new constellation approx. every
2160 yrs.
28. Zodiac Constellations
29. Zodiac Dates
30. Seasons on Earth
Earths seasons are caused by 2 factors:
Earths revolution around the Sun during the time span of 365.25
days
Tilted axis of approximately 23.5
The amount of solar energy Earths northern and southern hemispheres
receive at different times of the year continuously changes during
a revolution.
31. 32. Seasons on Earth
Winter Solstice
December 21/22
SDR 23.5 S (Tropic of Capricorn)
10 hrs. day, 14 hrs. night
Spring (Vernal) Equinox
March 20/21
SDR Equator
12 hrs. day, 12 hrs. night
Summer Solstice
June 21/22
SDR 23.5 N(Tropic of Cancer)
14 hrs. day, 10 hrs. night
Fall (Autumnal) Equinox
September 22/23
SDR Equator
12 hrs. day, 12 hrs. night
33. 34. Seasons on Earth
There is only a varying angle of incidence of the Suns rays from
season to season.
We receive more energy from the Sun when its shining on Earths
surface at a steeper angle of incidence.