CHAPTER 1 – CHAPTER 1 – Discoveringthe Night Sky

•Constellations•Celestial Sphere•Movements of the Earth•Moon Phases•Eclipses

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?

In You will discover…• how astronomers organize the night sky to help them how 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 the how the tilt of the Earth’s axis of rotation and the Earth’s motion around the Sun combine to create the Earth’s motion around the Sun combine to create the seasonsseasons

• that the Moon’s orbit around the Earth creates the that 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 was how the year is defined and how the calendar was developeddeveloped

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

THE SCALES OF THE 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.

ConstellationsConstellations

Groups of stars that form a pattern.Groups of stars that form a pattern.

In order to more easily locate objects in the sky, we In order to more easily locate objects in the sky, we divide the sky into regions named after familiar divide the sky into regions named after familiar patterns of stars called patterns 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.

Some Common Guides to Finding ConstellationsSome Common Guides to Finding Constellations

Using the “Big Dipper” as a guideUsing the “Big Dipper” as a guide

The “Summer Triangle”The “Summer Triangle”

The “Winter Triangle”The “Winter Triangle”

The Celestial SphereThe Celestial Sphere

North Celestial PoleNorth Celestial PoleSouth Celestial PoleSouth Celestial PoleCelestial EquatorCelestial EquatorDeclination (like latitude on Earth)Declination (like latitude on Earth)Right Ascension (like longitude on Earth)Right Ascension (like longitude on Earth)

Astronomers describe the universe as an imaginary Astronomers describe the universe as an imaginary sphere surrounding the earth on which all objects in sphere surrounding the earth on which all objects in the sky can be located, called the the sky can be located, called the CELESTIAL SPHERECELESTIAL SPHERE..

As viewed from Earth, the As viewed from Earth, the celestial sphere appears to rotate celestial sphere appears to rotate around two axis points, the around two axis points, the north north and south celestial polesand south celestial poles, which , which are located directly above the are located directly above the Earth’s poles.Earth’s poles.

Between these is the Between these is the celestial celestial equator,equator, which divides the which divides the celestial sphere into northern and celestial sphere into northern and southern hemispheres.southern hemispheres.

We define the position of an We define the position of an object on the celestial sphere object on the celestial sphere using two coordinates, using two coordinates, right right ascensionascension and and declinationdeclination. .

The Celestial SphereThe Celestial Sphere

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.

Cyclic motions of the Sun and stars in our sky Cyclic 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 one ROTATION=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 around REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one the sun. It takes one year for the Earth to complete one revolution.revolution.

3.3. PRECESSION=the slow conical (top-like) motion of the PRECESSION=the slow conical (top-like) motion of the Earth’s axis of rotation. It takes 26,000 years for the Earth Earth’s axis of rotation. It takes 26,000 years for the Earth to complete one cycle of precession. to complete one cycle of precession.

The apparent westward motion of the Sun, The apparent westward motion of the Sun, Moon, and stars across our sky each day is Moon, and stars across our sky each day is caused by Earth’s caused by Earth’s rotationrotation..

At middle latitudes, we see the Sun, Moon, and many of the stars At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the first 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 Sun We 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.”

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

The Earth also The Earth also revolvesrevolves around the Sun, which changes our view of the stars. around the Sun, which changes our view of the stars.

From our perspective, the Sun From our perspective, the Sun appears to move through the appears to move through the stars along a special path stars along a special path called the called the eclipticecliptic..

From an outside view, we see the From an outside view, we see the Earth revolve around the Sun. Earth revolve around the Sun. We define the plane of the We define the plane of the Earth’s orbit as the Earth’s orbit as the ecliptic planeecliptic plane..

We can see how different stars appear at different times of day by looking at We 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 Earth the 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 away facing 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

The constellations through which the Sun moves during the year as it The 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.

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

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

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

The seasons we experience are linked to the motion of the Sun along the The 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.

The Sun rises at different points along the horizon at different times of the year The 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.

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

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

The Lunar CycleThe Lunar Cycle

Phases of the Moon.Phases of the Moon.

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 the the 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 29½ Moon’s phases. This phase cycle is called the synodic period and is 29½ days long.days long.

One common misconception is One common misconception is that the Moon is only visible at that the Moon is only visible at night. However, the time of night. However, the time of day in which the Moon is in day in which the Moon is in our sky varies depending on our sky varies depending on its phase. This picture clearly its phase. This picture clearly displays the Moon, visible displays the Moon, visible

during the day.during the day.

A A synodic month synodic month is the time it takes for the Moon to orbit the Earth with is 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.

A A sidereal month sidereal month is the time it takes for the Moon to orbit the Earth with is 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 times The two times are different are different because the because the Earth moves in Earth moves in its orbit around its orbit around the Sun as the the Sun as the Moon moves in Moon moves in its orbit around its orbit around the Earth.the Earth.

EclipsesEclipses

There are two kinds of eclipses –There are two kinds of eclipses – Solar Eclipse – when the Moon is directly Solar Eclipse – when the Moon is directly

between the Sun and the Earth – the shadow between 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 directly Lunar Eclipse – when the Earth is directly between the Sun and the Moon – the Moon between the Sun and the Moon – the Moon enters the shadow of the Earth.enters the shadow of the Earth.

Solar EclipseSolar Eclipse

Solar EclipseSolar Eclipse

Lunar EclipseLunar Eclipse

Lunar EclipseLunar Eclipse

During a new or full moon phase, when the Moon, Sun, and Earth are During 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 the aligned, the Moon may enter the shadow of the Earth, or the shadow of the Moon may reach the Earth, creating eclipses. However, these eclipses do Moon may reach the Earth, creating eclipses. However, these eclipses do not occur during every full or new moon because the Moon’s orbit is tilted not 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.

THE THREE TYPES OF LUNAR ECLIPSESTHE THREE TYPES OF LUNAR ECLIPSES

PENUMBRAL = PENUMBRAL = the the Moon appears dimmed.Moon appears dimmed.

PARTIAL = PARTIAL = part of the part of the Moon enters the umbra of Moon enters the umbra of the Earth’s shadow and is the Earth’s shadow and is darkened.darkened.

TOTAL = TOTAL = all of the Moon all of the Moon enters the Earth’s shadow enters the Earth’s shadow and becomes a reddish and becomes a reddish color, only lit from light color, only lit from light bending around the bending around the Earth’s atmosphere.Earth’s atmosphere.

During a total lunar eclipse, the Moon moves in and out of During 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.

SOLAR ECLIPSES occur when the moon’s shadow reaches the earth.

Unlike lunar Unlike lunar eclipses, eclipses, solar solar eclipses eclipses occur at occur at specific specific places on places on the Earth, the Earth, indicated by indicated by the arrow.the arrow.

1.1. If you are located where the umbra of the Moon’s If 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 covered during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona by the Moon, revealing the faint solar corona surrounding the Sun.surrounding the Sun.

2.2. Those just outside of this region where the Those just outside of this region where the penumbra of the Moon’s shadow reaches will only penumbra of the Moon’s shadow reaches will only see a partial solar eclipse, during which only part of see 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 far Sometimes eclipses occur when the Moon is too far away from the Earth to completely cover the Sun in away from the Earth to completely cover the Sun in our sky. When this occurs, the Moon appears in the our sky. When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light center 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

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 the The tilt of the Earth’s rotation axis with respect to the

ecliptic.ecliptic.

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.