CHAPTER 1 – Discovering the 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 locate objects that the Earth’s spin on its axis causes day and night how the tilt of the Earth’s axis of rotation and the Earth’s motion around the Sun combine to create the seasons that the Moon’s orbit around the Earth creates the phases of the Moon and lunar and solar eclipses how the year is defined and how the calendar was developed

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: POWER DECIMAL NAME METRIC PREFIX 103 1000 one thousand Kilo 109 1,000,000,000 one billion Giga 10-6 0.000001 one millionth micro

THE SCALES OF THE 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.

Constellations Groups of stars that form a pattern.

In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars called constellations. 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 Constellations Using the “Big Dipper” as a guide

The “Summer Triangle”

The “Winter Triangle”

The Celestial Sphere North Celestial Pole South Celestial Pole Celestial Equator Declination (like latitude on Earth) Right Ascension (like longitude on Earth)

Astronomers describe the universe as an imaginary sphere surrounding the earth on which all objects in the sky can be located, called the CELESTIAL SPHERE. As viewed from Earth, the celestial sphere appears to rotate around two axis points, the north and south celestial poles, which are located directly above the Earth’s poles. Between these is the celestial equator, which divides the celestial sphere into northern and southern hemispheres. We define the position of an object on the celestial sphere using two coordinates, right ascension and declination.

The Celestial Sphere

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

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

The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation. 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 eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon. We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”

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

The Earth also revolves around the Sun, which changes our view of the stars. From our perspective, the Sun appears to move through the stars along a special path called the ecliptic. From an outside view, we see the Earth revolve around the Sun. We define the plane of the Earth’s orbit as the ecliptic plane.

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 facing the Sun is experiencing “day,” while the side of the Earth turned away from the Sun is experiencing “night.” MARCH SEPTEMBER

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

Constellations of the Zodiac Pisces – Two Fish Aries – The Ram Taurus – The Bull Gemini – The Twins Cancer – The Crab Leo – The Lion Virgo – The Maiden

More Constellations of the Zodiac Libra – The Scales (Balance) Scorpius – The Scorpion Ophiuchus – The Serpent Holder Sagittarius – The Archer Capricornus – The Goat Aquarius – The Water Carrier

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 sunlight (summer), while other parts of the Earth receive sunlight which is more spread out (winter).

The seasons we experience are linked to the motion of the Sun along the 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 and also peaks at different heights.

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 precession. Over a period of 26,000 years, the Earth’s rotation axis slowly moves in a circular motion.

This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star 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 this location.

The Lunar Cycle Phases of the Moon.

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

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

A synodic month is the time it takes for the Moon to orbit the Earth with respect to the Sun and is 29½ days long. A sidereal month is the time it takes for the Moon to orbit the Earth with respect to the stars and is 27.3 days long. The two times are different because the Earth moves in its orbit around the Sun as the Moon moves in its orbit around the Earth.

Eclipses There are two kinds of eclipses – Solar Eclipse – when the Moon is directly between the Sun and the Earth – the shadow of the Moon falls on the Earth. Lunar Eclipse – when the Earth is directly between the Sun and the Moon – the Moon enters the shadow of the Earth.

Solar Eclipse

Solar Eclipse

Lunar Eclipse

Lunar Eclipse

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 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 by 5 with respect to the Earth-Sun (ecliptic) plane.

THE THREE TYPES OF LUNAR ECLIPSES PENUMBRAL = the Moon appears dimmed. PARTIAL = part of the Moon enters the umbra of the Earth’s shadow and is darkened. TOTAL = all of the Moon enters the Earth’s shadow and becomes a reddish color, only lit from light bending around the Earth’s atmosphere. During a total lunar eclipse, the Moon moves in and out of the umbra of the Earth’s shadow.

SOLAR ECLIPSES occur when the moon’s shadow reaches the earth. Unlike lunar eclipses, solar eclipses occur at specific places on the Earth, indicated by the arrow.

If you are located where the umbra of the Moon’s shadow reaches, you will see a total solar eclipse, during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona surrounding the Sun. Those just outside of this region where the penumbra of the Moon’s shadow reaches will only see a partial solar eclipse, during which only part of the disk of the Sun is covered by the Moon. Sometimes eclipses occur when the Moon is too far away from the Earth to completely cover the Sun in our sky. When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light 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 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.