Discovering the night sky

1. CHAPTER 1 –CHAPTER 1 –
Discovering
the Night Sky
•Constellations
•Celestial Sphere
•Movements of the
Earth
•Moon Phases
•Eclipses

2. WHAT DO YOU THINK?
 Is the North Star—Polaris—the brightest star in
the night sky?
 Do astronomers regard constellations as the
familiar patterns of stars in the sky?
 What causes the seasons?
 When is the Earth closest to the Sun?
 How many zodiac constellations are there?
 Does the Moon have a dark side that we never
see from Earth?
 Is the Moon ever visible during the daytime?

3. In You will discover…
• how astronomers organize the night sky to help themhow astronomers organize the night sky to help them
locate objectslocate objects
• that the Earth’s spin on its axis causes day and nightthat the Earth’s spin on its axis causes day and night
• how the tilt of the Earth’s axis of rotation and thehow the tilt of the Earth’s axis of rotation and the
Earth’s motion around the Sun combine to create theEarth’s motion around the Sun combine to create the
seasonsseasons
• that the Moon’s orbit around the Earth creates thethat the Moon’s orbit around the Earth creates the
phases of the Moon and lunar and solar eclipsesphases of the Moon and lunar and solar eclipses
• how the year is defined and how the calendar washow the year is defined and how the calendar was
developeddeveloped

4. The universe is huge, and the sizes and distances of
objects in the universe vary greatly. Therefore we use
scientific notation, which involves powers of ten notation to
describe numbers much smaller or much greater than 1.
Some common examples of powers of ten:Some common examples of powers of ten:
POWERPOWER DECIMALDECIMAL NAMENAME METRIC PREFIXMETRIC PREFIX
101033
10001000 one thousandone thousand KiloKilo
101099
1,000,000,0001,000,000,000 one billionone billion GigaGiga
1010-6-6
0.0000010.000001 one millionthone millionth micromicro

5. THE SCALES OFTHE SCALES OF
THE UNIVERSETHE UNIVERSE
The range of objects
we study are from the
extremely small
subatomic particles,
to objects which are
gigantic, such as a
galaxy or the size of
the known universe
itself.
Each division up the
line indicates an
increase in size by
100,000.

6. ConstellationsConstellations
 Groups of stars that form a pattern.Groups of stars that form a pattern.

7. In order to more easily locate objects in the sky, weIn order to more easily locate objects in the sky, we
divide the sky into regions named after familiardivide the sky into regions named after familiar
patterns of stars calledpatterns of stars called constellationsconstellations..
Ancient constellations were
imaginary pictures outlined
by familiar patterns of stars.
Modern astronomers divide the sky into
88 official constellations or regions of
space, many of which contain the
ancient star patterns.

8. Some Common Guides to Finding ConstellationsSome Common Guides to Finding Constellations
Using the “Big Dipper” as a guideUsing the “Big Dipper” as a guide

9. The “Summer Triangle”The “Summer Triangle”

10. The “Winter Triangle”The “Winter Triangle”

11. The Celestial SphereThe Celestial Sphere
 North Celestial PoleNorth Celestial Pole
 South Celestial PoleSouth Celestial Pole
 Celestial EquatorCelestial Equator
 Declination (like latitude on Earth)Declination (like latitude on Earth)
 Right Ascension (like longitude on Earth)Right Ascension (like longitude on Earth)

12. Astronomers describe the universe as an imaginaryAstronomers describe the universe as an imaginary
sphere surrounding the earth on which all objects insphere surrounding the earth on which all objects in
the sky can be located, called thethe sky can be located, called the CELESTIAL SPHERECELESTIAL SPHERE..
 As viewed from Earth, theAs viewed from Earth, the
celestial sphere appears to rotatecelestial sphere appears to rotate
around two axis points, thearound two axis points, the northnorth
and south celestial polesand south celestial poles, which, which
are located directly above theare located directly above the
Earth’s poles.Earth’s poles.
Between these is theBetween these is the celestialcelestial
equator,equator, which divides thewhich divides the
celestial sphere into northern andcelestial sphere into northern and
southern hemispheres.southern hemispheres.
We define the position of anWe define the position of an
object on the celestial sphereobject on the celestial sphere
using two coordinates,using two coordinates, rightright
ascensionascension andand declinationdeclination..

13. The Celestial SphereThe Celestial Sphere

14. Motions of the EarthMotions of the Earth
 Rotation – spins on axis.Rotation – spins on axis.
 Revolution – goes around the Sun.Revolution – goes around the Sun.
 Precession – Earth’s axis wobbles.Precession – Earth’s axis wobbles.

15. Cyclic motions of the Sun and stars in our skyCyclic motions of the Sun and stars in our sky
are due to motions of the Earthare due to motions of the Earth
1.1. ROTATION=the spin of the Earth on its axis. It takes oneROTATION=the spin of the Earth on its axis. It takes one
day for the Earth to complete one rotation.day for the Earth to complete one rotation.
2.2. REVOLUTION=the movement of the Earth in orbit aroundREVOLUTION=the movement of the Earth in orbit around
the sun. It takes one year for the Earth to complete onethe sun. It takes one year for the Earth to complete one
revolution.revolution.
3.3. PRECESSION=the slow conical (top-like) motion of thePRECESSION=the slow conical (top-like) motion of the
Earth’s axis of rotation. It takes 26,000 years for the EarthEarth’s axis of rotation. It takes 26,000 years for the Earth
to complete one cycle of precession.to complete one cycle of precession.

16. The apparent westward motion of the Sun,The apparent westward motion of the Sun,
Moon, and stars across our sky each day isMoon, and stars across our sky each day is
caused by Earth’scaused by Earth’s rotationrotation..
At middle latitudes, we see the Sun, Moon, and many of the starsAt middle latitudes, we see the Sun, Moon, and many of the stars
first come into view moving upward, rising at some point along thefirst come into view moving upward, rising at some point along the
eastern horizon. Then, they appear to arc across the sky. Finally,eastern horizon. Then, they appear to arc across the sky. Finally,
they disappear somewhere along the western horizon.they disappear somewhere along the western horizon.
We generalize this motion to make statements such as, “The SunWe generalize this motion to make statements such as, “The Sun
rises in the east and sets in the west.”rises in the east and sets in the west.”

17. Different parts of the world experience different times of day as the EarthDifferent parts of the world experience different times of day as the Earth
rotates.rotates.
TIME ZONES which can be used to calculate the time of day in any givenTIME ZONES which can be used to calculate the time of day in any given
part of the world.part of the world.

18. The Earth alsoThe Earth also revolvesrevolves around the Sun, which changes our view of thearound the Sun, which changes our view of the
stars.stars.
From our perspective, the SunFrom our perspective, the Sun
appears to move through theappears to move through the
stars along a special pathstars along a special path
called thecalled the eclipticecliptic..
From an outside view, we see theFrom an outside view, we see the
Earth revolve around the Sun.Earth revolve around the Sun.
We define the plane of theWe define the plane of the
Earth’s orbit as theEarth’s orbit as the ecliptic planeecliptic plane..

19. We can see how different stars appear at different times of day by looking atWe can see how different stars appear at different times of day by looking at
the position of the Sun against the backdrop of stars. The side of the Earththe position of the Sun against the backdrop of stars. The side of the Earth
facing the Sun is experiencing “day,” while the side of the Earth turned awayfacing the Sun is experiencing “day,” while the side of the Earth turned away
from the Sun is experiencing “night.”from the Sun is experiencing “night.”
SEPTEMBER MARCH

20. The constellations through which the Sun moves during the year as itThe constellations through which the Sun moves during the year as it
travels along the ecliptic are called the 13 constellations of the zodiac.travels along the ecliptic are called the 13 constellations of the zodiac.

21. Constellations of the ZodiacConstellations of the Zodiac
1.1. Pisces – Two FishPisces – Two Fish
2.2. Aries – The RamAries – The Ram
3.3. Taurus – The BullTaurus – The Bull
4.4. Gemini – The TwinsGemini – The Twins
5.5. Cancer – The CrabCancer – The Crab
6.6. Leo – The LionLeo – The Lion
7.7. Virgo – The MaidenVirgo – The Maiden

22. More Constellations of the ZodiacMore Constellations of the Zodiac
8.8. Libra – The Scales (Balance)Libra – The Scales (Balance)
9.9. Scorpius – The ScorpionScorpius – The Scorpion
10.10. Ophiuchus – The Serpent HolderOphiuchus – The Serpent Holder
11.11. Sagittarius – The ArcherSagittarius – The Archer
12.12. Capricornus – The GoatCapricornus – The Goat
13.13. Aquarius – The Water CarrierAquarius – The Water Carrier

23. Seasons are caused because the Earth’s axis is tilted and as the EarthSeasons are caused because the Earth’s axis is tilted and as the Earth
revolves around the Sun, different parts of the Earth receive more directrevolves around the Sun, different parts of the Earth receive more direct
sunlight (summer), while other parts of the Earth receive sunlight which issunlight (summer), while other parts of the Earth receive sunlight which is
more spread out (winter).more spread out (winter).

24. The seasons we experience are linked to the motion of the Sun along theThe seasons we experience are linked to the motion of the Sun along the
celestial sphere.celestial sphere.
The point of the Sun’s path
farthest north on the celestial
sphere is called the summer
solstice (JUN 21), while the point
of the ecliptic farthest south is
called the winter solstice (DEC
21).
The two points on the ecliptic
where the Sun crosses the
celestial equator are called
equinoxes. During the vernal
equinox (MAR 21), the Sun is
moving north, while during the
autumnal equinox (SEPT 21), the
Sun is moving south.
Remember that the seasonal
names of the equinoxes and the
solstices refer to seasons in the
NORTHERN hemisphere. The
seasons occurring in the
SOUTHERN hemisphere are
exactly opposite.

25. The Sun rises at different points along the horizon at different times of the yearThe Sun rises at different points along the horizon at different times of the year
and also peaks at different heights.and also peaks at different heights.

27. Gravitational forces of the Sun and the Moon pulling on the Earth as itGravitational forces of the Sun and the Moon pulling on the Earth as it
rotates causes the Earth to undergo a top-like motion calledrotates causes the Earth to undergo a top-like motion called precessionprecession..
Over a period of 26,000 years, the Earth’s rotation axis slowly moves in aOver a period of 26,000 years, the Earth’s rotation axis slowly moves in a
circular motion.circular motion.

28. This precession causes the position of the North Celestial Pole to slowlyThis precession causes the position of the North Celestial Pole to slowly
change over time. Today, the North Celestial Pole is near the starchange over time. Today, the North Celestial Pole is near the star
Polaris, which we call the “North Star.” However, in 3000 BC, ThubanPolaris, which we call the “North Star.” However, in 3000 BC, Thuban
was close to the North Celestial Pole and in 14,000AD, Vega will be inwas close to the North Celestial Pole and in 14,000AD, Vega will be in
this location.this location.

29. The Lunar CycleThe Lunar Cycle
 Phases of the Moon.Phases of the Moon.

30. Another familiar cycle is the lunar cycle. When the Moon orbits the Earth,Another familiar cycle is the lunar cycle. When the Moon orbits the Earth,
the amount of the side facing the Earth that is lit changes, creating thethe amount of the side facing the Earth that is lit changes, creating the
Moon’s phases. This phase cycle is called the synodic period and is 29½Moon’s phases. This phase cycle is called the synodic period and is 29½
days long.days long.

31. One common misconception isOne common misconception is
that the Moon is only visible atthat the Moon is only visible at
night. However, the time ofnight. However, the time of
day in which the Moon is inday in which the Moon is in
our sky varies depending onour sky varies depending on
its phase. This picture clearlyits phase. This picture clearly
displays the Moon, visibledisplays the Moon, visible
during the day.during the day.

32. AA synodic monthsynodic month is the time it takes for the Moon to orbit the Earth withis the time it takes for the Moon to orbit the Earth with
respect to the Sun and is 29½ days long.respect to the Sun and is 29½ days long.
AA sidereal monthsidereal month is the time it takes for the Moon to orbit the Earth withis the time it takes for the Moon to orbit the Earth with
respect to the stars and is 27.3 days long.respect to the stars and is 27.3 days long.
The two timesThe two times
are differentare different
because thebecause the
Earth moves inEarth moves in
its orbit aroundits orbit around
the Sun as thethe Sun as the
Moon moves inMoon moves in
its orbit aroundits orbit around
the Earth.the Earth.

33. EclipsesEclipses
 There are two kinds of eclipses –There are two kinds of eclipses –
 Solar Eclipse – when the Moon is directlySolar Eclipse – when the Moon is directly
between the Sun and the Earth – the shadowbetween the Sun and the Earth – the shadow
of the Moon falls on the Earth.of the Moon falls on the Earth.
 Lunar Eclipse – when the Earth is directlyLunar Eclipse – when the Earth is directly
between the Sun and the Moon – the Moonbetween the Sun and the Moon – the Moon
enters the shadow of the Earth.enters the shadow of the Earth.

34. Solar EclipseSolar Eclipse

35. Solar EclipseSolar Eclipse

36. Lunar EclipseLunar Eclipse

37. Lunar EclipseLunar Eclipse

38. During a new or full moon phase, when the Moon, Sun, and Earth areDuring a new or full moon phase, when the Moon, Sun, and Earth are
aligned, the Moon may enter the shadow of the Earth, or the shadow of thealigned, the Moon may enter the shadow of the Earth, or the shadow of the
Moon may reach the Earth, creating eclipses. However, these eclipses doMoon may reach the Earth, creating eclipses. However, these eclipses do
not occur during every full or new moon because the Moon’s orbit is tiltednot occur during every full or new moon because the Moon’s orbit is tilted
by 5by 5°° with respect to the Earth-Sun (ecliptic) plane.with respect to the Earth-Sun (ecliptic) plane.

39. THE THREE TYPES OF LUNAR ECLIPSESTHE THREE TYPES OF LUNAR ECLIPSES
PENUMBRAL =PENUMBRAL = thethe
Moon appears dimmed.Moon appears dimmed.
PARTIAL =PARTIAL = part of thepart of the
Moon enters the umbra ofMoon enters the umbra of
the Earth’s shadow and isthe Earth’s shadow and is
darkened.darkened.
TOTAL =TOTAL = all of the Moonall of the Moon
enters the Earth’s shadowenters the Earth’s shadow
and becomes a reddishand becomes a reddish
color, only lit from lightcolor, only lit from light
bending around thebending around the
Earth’s atmosphere.Earth’s atmosphere.
During a total lunar eclipse, the Moon moves in and out ofDuring a total lunar eclipse, the Moon moves in and out of
the umbra of the Earth’s shadow.the umbra of the Earth’s shadow.

41. SOLAR ECLIPSES occur when the moon’s shadow reaches the earth.
Unlike lunarUnlike lunar
eclipses,eclipses,
solarsolar
eclipseseclipses
occur atoccur at
specificspecific
places onplaces on
the Earth,the Earth,
indicated byindicated by
the arrow.the arrow.

42. 1.1. If you are located where the umbra of the Moon’sIf you are located where the umbra of the Moon’s
shadow reaches, you will see a total solar eclipse,shadow reaches, you will see a total solar eclipse,
during which the entire disk of the Sun is coveredduring which the entire disk of the Sun is covered
by the Moon, revealing the faint solar coronaby the Moon, revealing the faint solar corona
surrounding the Sun.surrounding the Sun.
2.2. Those just outside of this region where theThose just outside of this region where the
penumbra of the Moon’s shadow reaches will onlypenumbra of the Moon’s shadow reaches will only
see a partial solar eclipse, during which only part ofsee a partial solar eclipse, during which only part of
the disk of the Sun is covered by the Moon.the disk of the Sun is covered by the Moon.
3.3. Sometimes eclipses occur when the Moon is too farSometimes eclipses occur when the Moon is too far
away from the Earth to completely cover the Sun inaway from the Earth to completely cover the Sun in
our sky. When this occurs, the Moon appears in theour sky. When this occurs, the Moon appears in the
center and a thin ring, or “annulus,” of lightcenter and a thin ring, or “annulus,” of light
surrounds it. These are called annular eclipses.surrounds it. These are called annular eclipses.
TOTAL ECLIPSE
ANNULAR ECLIPSE

44. WHAT DID YOU KNOW?
 Is the North Star—Polaris—the brightest star in
the night sky?
 No. Polaris is of medium brightness.
 Do astronomers regard constellations as the
familiar patterns of stars in the sky?
 Sometimes. However, a constellation is formally an
entire region of the celestial sphere.
 What causes the seasons?
 The tilt of the Earth’s rotation axis with respect to theThe tilt of the Earth’s rotation axis with respect to the
ecliptic.ecliptic.

45. WHAT DID YOU KNOW?
 When is the Earth closest to the Sun?
 January 3 of each year.
 How many zodiac constellations are there?
 13
 Does the Moon have a dark side that we never
see from Earth?
 Half of the Moon is always dark, but when we see
less than a full Moon, we are seeing part of the dark
side.
 Is the Moon ever visible during the daytime?
 The Moon is visible at some time during daylight
hours almost every day of the year.