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.