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The Night Sky

Lecture 25

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Outline

The night sky - overview

Why the sky changes During the night

From night to night

Some definitions Celestial sphere, ecliptic, zodiac, zenith, meridian

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Things to see in the night sky

The Moon & Planets

Stars Constellations

Asterisms (groupings of stars) e.g. the Pleiades)

Binaries

Variable Stars

Sky map – north view

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Sky map – south view

Sky map – south view

http://www.skyviewcafe.com/skyview.php

Tonight at 8:00 pm

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In-Class Question

Roughly how many stars are visible to the unaided (eye over the entire sky), that is, how many could you seeA. 6B. 600C. 6,000D. 60,000E. 600,000

27,0007.5

8,8006.5

2,8005.5

9004.5

Total # of stars

Visual Magnitude

!

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With binoculars or a telescope

Object Example

Globular cluster - M13 in Hercules

Open clusters - Pleiadesh and Perseus

Planetary Nebula - Ring Nebula

Galaxy - Andromeda

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M13 – Globular Cluster

h and Perseus

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Pleiades Star Cluster

Ring Nebula (M57)

HST Image

Ground Images

Above - rescaled

Deeper exposure

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Andromeda Galaxy (M31)

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But where to look?

Need to know where an object is on the sky (coordinates or it location relative to some reference stars or constellations)

And what time that part of the sky is viewable, e.g. the sun is not up. Time of night and Time of year

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How do we know where and when to look?

The major motions that affect the night sky are:

Rotation of the earth Causes the stars to move through the sky

at night.

Revolution of the earth around the sun Causes different stars to be available at

different times of the year.

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N

Not to scale!

Pegasus &Andromeda

Lyra &Hercules

Leo &Coma

Auriga &Orion

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Real proportions

If the sun were the volleyball: ~ 8 in = 20 cm in diameter

The earth is smaller than a pea: ~ 1.8 mm in diameter

The earth-sun distance is: 22 m (about 70 feet)

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Cycles of the Sky

A class demo, wherein the part of the sun is play by a tennis ball

while the earth is played by a volleyball,

and the students are, of course, stars.

(And I get to play Atlas!)

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N

Not to scale!

Pegasus &Andromeda

Lyra &Hercules

Leo &Coma

Auriga &Orion

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In-Class Question What causes the stars to move through the

sky at night?

A. The rotation of the sky about the Earth

B. The rotation of the Earth

C. The revolution of the Earth about the Sun

D. Magic

E. The planets drag the stars along with them

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The Celestial Sphere The celestial sphere is

the vast hollow sphere on which the stars appear fixed.

The celestial equator is define by extending the earth’s equator outward.

The N & S poles of the celestial sphere correspond to the earth’s poles.

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The Ecliptic The ecliptic is the

apparent path of the sun through the sky.

It is also the plane of the earth’s orbit about the sun on the celestial sphere.

Note: The ecliptic is tilted w.r.t. the earth’s equatorial plane by 23.5o.

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The Zodiac

The zodiac is a band of celestial sphere which represents the path of the planets, the moon and the sun.

Extends ~8o to either side of the ecliptic.

In astrology the zodiac is divided into 12 equal parts called signs, each bearing the name of a constellation.

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Astrology is NOT a science!

Propagates the claim that a person’s life is determined by the position of the sun, moon, and planets at birth.

This notion is patently false, and potentially harmful.

Astrology is neither a science nor a religion.

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Zenith

The zenith is the point on the celestial sphere that is directly above the observer.

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Meridian

The meridian is the great circle passing through the two poles of the celestial sphere and the observer’s zenith.