Discovering the Night Sky

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.

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

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”

Astronomers describe the universe as an imaginary sphere surrounding the earth, called the CELESTIAL SPHERE. As viewed from Earth, the celestial sphere rotates around 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.

Cyclic motions of the Earth ROTATION= REVOLUTION= 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.”

The Earth also revolves around the Sun, which changes our view of the stars. From our perspective, the Suns path is ecliptic. From an outside view, we see the Earth revolve around the Sun. the plane of the Earth’s orbit in on 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 that revolve with the Sun as it moves during the year along the ecliptic are called the 13 constellations of the zodiac.

Constellations of the Zodiac Pisces – Two Fish Scorpius – The Scorpion Aries – The Ram Ophiuchus – The Serpent Holder Taurus – The Bull Gemini – The Twins Sagittarius – The Archer Cancer – The Crab Leo – The Lion Capricornus – The Goat Virgo – The Maiden Libra – The Scales (Balance) Aquarius – The Water Carrier

Seasons are caused b/c Earth’s axis is tilted and as it revolves around the Sun, different parts of Earth receive different amounts of sunlight. more direct sunlight (summer) more spread out sunlight (winter).

The Sun rises at different points along the horizon at different times of the year and also peaks at different heights. The zenith is considered the highest most point in the sky or celestial sphere located directly above an observer.

Gravitational forces of Sun and Moon pulling on Earth as it rotates causes it 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