Að uppgötva næturhimininn

Hvað heldur þú?

• Hve björt er norðurstjarnan, Pólstjarnan, í samanburði við aðrar stjörnur?

• Eru stjörnumerkin aðeins goðfræðlegar myndir á himninum?

• Hvað veldur árstíðaskiptum?• Hvað eru stjörnumerkin mörg?• Hvenær, er tunglið sýnilegt á daginn?

Stjörnumerkin auðvelda staðsetningu stjarnanna

Notaðu Stóra

björninn á norðurhveli himins tio að finna

aðra hópa stjarnanna

Notaðu vetrar þríhyrningin til að finna

suðurhimins stjörnumerkin

á vetrar-kvöldm

Notaðu Sumar Þríhyrninginn til að finna suðræn stjörnumerki á sumarkvöldum

Hornið milli stjarna á himninum er notað til að áætla fjarlægðir

Hornfjarlægð milli stjarna á himninum eru mæld í gráðum

Ímyndað Himinhvolf

sem umlykur

Jörðina er notað til að

túlka himininn

Elements of the equatorial coordinate system on the celestial sphere

• Vernal Equinox: The position of the Sun on the first day of spring

• Right Ascension: How far east of the Vernal Equinox an object is located

• Celestial Equator: The line separating the celestial sphere into northern and southern halves.

• Declination: How far above or below the celestial equator an object is located.

Earth’s rotation

causes the Sun and stars to

appear to move

Motion of Stars at the North Pole

Motion of Stars at the Equator

What about the Sun? It seems to move across the celestial sphere on a path called the ecliptic.

Equinox positionSummer solstice

Winter solstice

Position of the “noon-time” Sun when photographed every two weeks over a year

The “altitude” of the Sun impacts how much energy is received on Earth resulting in seasons.

The seasons are caused by the tilt of the Earth’s axis of rotation

In other words ...

• The Earth’s rotation on its axis determines the length of the day

• The Earth’s orbit around the Sun (revolution) determines the length of the year

• The Earth’s tilt with respect to the ecliptic causes the seasons.

Precession is a slow, circular motion of the Earth’s axis of rotation

Precession causes the position of the

NCP to change over a period of 26,000 years.

The changing phases of the Moon originally inspired the concept of the month

Moon Phase Names

• New Moon• Waxing Crescent• First Quarter• Waxing Gibbous• Full Moon

• Full Moon• Waning Gibbous• Third Quarter• Waning Crescent• New Moon

Although the Moon is always lit from the Sun, we see

different amounts of the lit portion from

Earth depending on

where the Moon is

located in its month-long

orbit.

How long does it take to go from new moon to new moon?

27.3 days 29.5 days

How long does it take the Moon to

orbit Earth?lunar month synodic month

Eclipses occur only when the Moon crosses the ecliptic during the new or full phase

Types of Lunar Eclipses

• total eclipse

• penumbral eclipse

• partial eclipse

Solar Eclipse

Annular Eclipse

Partial Eclipse

Total Eclipse

What did you think?• How bright is the North Star, Polaris, compared to other stars?

Polaris is a star of medium brightness compared to other stars visible to the naked eye.

• Are constellations just mythic figures in the sky? A constellation is an entire region of the celestial sphere and all that stars

in it.

• What causes the seasons? The tilt of Earth’s rotational axis causes the seasons.

• How many zodiac constellations are there? There are 13 zodiac constellations, the “unknown one is Ophiuchus.

• When, if ever, is the Moon visible during the daytime? The Moon is visible during the day for half of the lunar cycle of phases.

Self-Check1. Describe the nature and value of constellations.

2. Define the elements of the equatorial coordinate system on the celestial sphere.

3. Define two solstices and two equinoxes; explain the orientation of the ecliptic on the celestial sphere and how it produces seasons on the Earth.

4. Describe what precession is, what effect it has on our observations of stars, and why it occurs.

5. Explain by drawing a diagram showing how lunar phases are controlled by the relative positions of the Sun and the Moon.

6. Explain when and why solar and lunar eclipses occur and why there are not such eclipses every month.