1445
Introductory Astronomy I
Chapter 1
The Night Sky
and Motions of Sun, Earth and Moon
R. S. Rubins
Fall, 2010
1
The Geocentric Universe
In the ancient idea of a geocentric universe, the Earth was
assumed to be at the center of the universe.
Outside the Earth, the Sun and the Moon were the most
important celestial objects.
We now know that the importance to us of the tiny Moon,
lies only in its proximity, just 240,000 miles from the Earth.
•
Behind the Moon were the fixed stars, which appeared to
move together around the Earth in a regular motion.
Among the stars were found the planets, following irregular
paths, but never straying far from the Sun’s path, which is
known as the ecliptic.
2
The Planets
The ancient Greeks introduced ingenious, but complicated
ideas, to describe planetary motions about the Earth in a
manner in keeping with the geocentric model.
Their final model was that of Ptolemy (2nd century), which
held sway until the Copernican revolution of the 16th
century.
The Earth lies at 93 million miles (or 1 astronomical unit)
from the Sun, which is small distance compared to the 3
billion mile of the outermost (major) planet Neptune.
Both these distances are insignificant compared to the
distance the nearest star, Proxima Centauri, which is about
25 trillion miles (approximately 4 light-years) away.
3
How Many Stars?
•
A total of about 6000 stars can be seen by the unaided
human eye, although only about half at any one time.
•
However, about one half of these stars that we imagine to
be single are actually binary pairs; i.e. double stars, which
are very close together.
•
Thus, without realizing it, we actually see about 9000 stars.

There are estimated to be about 200 billion (2 x 1011) stars
in our galaxy, the Milky Way.

Since there are at least 50 billion galaxies in the visible
universe, there should be a total of more than 10 billion
trillion (1022) stars. although the vast majority cannot be
seen, even with the most powerful telescopes.
4
Practical Use of Astronomy
•
The time to plant seeds was predicted from
i. the positions of the constellations;
ii. the height of the noontime Sun.
•
Planning sea travel often depended on the tides, which
are influenced by the positions of the Moon and the Sun.
•
The positions of the Sun in the day and the
constellations at night were used for navigation at sea.
•
In particular, the North Star, Polaris, was very important
in navigation (in the northern hemisphere), because it
closely marks the direction of due north, and its altitude
in the sky gives the latitude from which it is observed.
5
Constellations
•
•
In popular usage, the term constellation is used to denote a
recognizable grouping of stars.
Astronomers have redefined the constellations as 88 regions of
the night sky, while referring to the groupings as asterisms .
The constellation Orion
popular usage
astronomers usage
6
The Big Dipper as a Guide

The two “pointer stars”
furthest from the handle
of the Big Dipper point to
Polaris (the North Star).

The next two stars point
in to Regulus, the
brightest star in the
constellation Leo.

The pattern may appear
upside-down because it
rotates about Polaris.
7
The Winter Triangle

The Winter Triangle
connects three bright
stars: Betelgeuse (in
Orion), Procyon (in
Canis Minor) and
Sirius (in Canis Major).

This triangle is almost
equilateral, but slightly
stretched in the
direction of Sirius.
8
The Summer Triangle

The Summer Triangle
connects three bright
stars: Vega (in Lyra),
Deneb (in Cygnus) and
Altair (in Aquila).

This triangle is
stretched in the
direction of Altair.
9
Celestial Sphere 1
•
The celestial sphere is an imaginary hollow sphere, with the
Earth at its center, to which all the stars seen in the night sky
appear to be fixed .
The motion of the stars in the night sky may be visualized as
a rotation of the celestial sphere from east to west about a
north-south axis.
•
The rotation is from east to west because the stars rise in the
east and set in the west.
•
The fixed stars are actually at widely varying distances, all
more than 4 light years (25 trillion miles) away, moving
relative to each other with motions that are not apparent to us.
•
As a result, changes in appearance of the constellations are
not apparent in a human life-span.
10
Celestial Sphere 2
Know the following:
North Celestial Pole
South Celestial Pole
Celestial Equator
Declination (latitude)
is measured from
the Celestial Equator.
Right Ascension (longitude)
is measured from the
Vernal Equinox (see below).
11
Australian View of the South Celestial Pole


In the Celestial Sphere picture, the stars all rotate from east to
west about the line through the celestial poles.
Thus, in Australia, near the Earth’s south pole, the stars all
appear to rotate about the South Celestial Pole.
12
The Apparent Motion of the Night Sky
Equator
West
USA
North Pole
West
• The stars appear to move from east to west as follows:
i. vertically downwards at the equator (if facing West);
ii. downwards and to the right in the USA (if facing West);
iii. from left to right at the north Pole;
iv. from right to left at the South Pole.
13
Celestial Sphere and Ecliptic 3

•
Geocentric view
Since the Earth is considered to be at rest at the center of the
Universe, the ecliptic is defined as the annual path of the Sun
around the celestial sphere.
Sun moves on the ecliptic.
23½ o is the angle
between the ecliptic and
the celestial equator.
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Celestial Sphere and Ecliptic 2
•
In the geocentric view, the plane of the ecliptic makes an
angle of 23½o, with the celestial equator.
15
The Ecliptic: Heliocentric View
•
In the heliocentric view, the ecliptic is defined as the path
of the Earth’s orbit around the Sun.
•
The Earth rotates from east to west, about an axis tilted by
23½o from the normal to the ecliptic plane.
•
23½o
North-south
rotational
axis.
Normal to
the ecliptic.
16
Equinoxes
•
Equinoxes (Latin “equal nights”) are those times of the year
in which day and night are of roughly equal length, which
occur when the Sun’s position on the ecliptic crosses the
celestial equator.
•
The vernal equinox occurs on about March 21, when the
Sun crosses the celestial equator heading north.
•
The autumnal equinox occurs on about September 22, when
the Sun crosses the celestial equator heading south.
•
Between the vernal and autumnal equinoxes, the days are
longer in the northern hemisphere, and the Sun is higher in
the sky at mid-day.
•
The reverse is true for the time between the autumnal and
vernal equinoxes.
17
Solstices and the Seasons
•
The summer solstice occurs on about June 21, when the
Sun reaches the point on the ecliptic furthest north from the
celestial equator.
•
In summer, the Sun rises in the NE and sets in the NW.
•
The winter solstice occurs on about December 21, when the
Sun reaches the point on the ecliptic furthest south from the
celestial equator.
•
In winter, the Sun rises in the SE and sets in the SW.
•
If the Earth’s rotation axis were not tilted, seasons (as we
know them) would not exist, and every night would last
roughly 12 hours.
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Equinoxes, Solstices and the Seasons 3
Summer
Winter
The Sun’s
daily path
19
The Seasons and the Earth’s Axis
•
The seasons result from both the 23½o tilt of the Earth’s
rotation axis and its orbit about the Sun.
20
Effect of the Changing Distance of the Sun
•
•
•
While the Sun’s distance from the Earth varies slightly
throughout the year, becoming closest on about January 3, it
has no noticeable effect on the climate.
The effect of the Sun being closer in the northern winter is
reduced by the fact that the southern hemisphere has a
higher percentage of oceans, which reflect heat and light
back into space more efficiently than do forested land
masses.
If the Earth’s orbit were very elliptical (like Mercury), then this
effect would be more pronounced, and if, in addition, the
Earth’s axis were not tilted, then the seasons would be
produced only by the varying distance of the Sun.
However, in the latter case, the seasons so produced, would
occur at the same time for both hemispheres.
21
The Earth’s Precessional Motion 1
•
The precessional motion of the Earth’s axis is a very slow
conical motion caused by the combined gravitational pulls of
the Sun and the Moon.
•
The motion is analogous to that of a spinning top.
•
Calculations have shown that without the presence of the
Moon, the 23½o tilt of the Earth’s rotation axis would not be
maintained, with wild swings in the tilt angle being the rule.
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The Earth’s Precessional Motion 2
•
During the precessional
period of about 26,000
years, the Earth’s northsouth axis traces out a
circle in the sky.
•
Presently, the celestial
North Pole points to within
a degree of Polaris, but in
the year 14,000, it will point
roughly towards Vega.
23
The Zodiac
•
•
•
•
On its apparent eastward journey around the ecliptic, the
Sun appears to pass through the twelve Constellations of
the Zodiac.
In 1930, astronomers added a thirteenth constellation –
Ophiuchus – which the Sun passes through between
December 1 and December 19 each year.
Over 2000 years ago when the pseudoscience of astrology
was introduced by the famous mathematician Euclid, a
person’s astrological sign was determined by where the Sun
was in the Zodiac on his/her birthday.
Because of the Earth’s precessional motion, our birthdays
are now one sign later than they were 2000 years ago.
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Traveling on Spaceship Earth
•
Although we imagine ourselves to be at rest, the Earth takes
part in the motions outlined below.
•
The Earth spins about its N-S axis, with a period of 1 day,
and a rotational speed varies from 1650 km/hr (1030 mi/hr)
at the equator to zero at the poles.
•
The Earth orbits the Sun with a 1 year period, and a speed
of above 100,000 km/hr (60,000 mi/hr).
•
Our solar system orbits the center of our galaxy with a 230
million year period, and a speed slightly of about 800,000
km/h (500,000 mi/hr).
•
Our galaxy orbits the mass-center of the Local Groupr of
galaxies, which in turn orbits the center of the Local (or
Virgo) Supercluster.
25
Siderial and Synodic Periods
•
A siderial period is a period measured with respect to the
distant stars.
•
A synodic period is the period measured from a planet (or
moon).
•
The solar day is the synodic day measured from Earth, which
is longer than the siderial day by about 4 min.
•
The lunar month is the synodic month measured from Earth,
which is longer than the siderial month by approximately 2.2
days.
•
The tropical year is the synodic year, measured between
successive vernal equinoxes, which is shorter than the
siderial year by about 20 minutes.
26
Solar and Sidereal Days
•
•
The solar day is the average time (24 hours) between
successive noon-times, as measured at 0o longitude in
Greenwich, England (the prime meridian).
The sidereal day is the time (23 hours 56 min.) taken for a
planet to make one complete revolution.
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Lunar and Sidereal Months
•
•
The synodic or lunar month is the time (approximately 29½
days) between identical phases of the moon; e.g. from full
moon to full moon.
The sidereal month is the time (approximately 27.3 days) it
takes the Moon to make one full orbit (360o) around the Earth.
28
The Year and the Calendar
•
Ancient astronomers realized that the year was roughly 365¼
days long.
•
In 47 BCE, Julius Caesar added an extra day every 4 years,
thus creating leap years of 366 days.
•
Pope Gregory XIII reformed the Julian calendar in 1582, leaving
out 10 days to get the seasons back on schedule, and
decreeing that only those century years divisible by 400 were to
be leap years.
•
The average Gregorian year differs by only one day in 3300
years from the tropical year.
•
With the modification that the years 4000, 8000, 12,000 and
16,000 are not to be leap years, the Gregorian system will not
have to be revised for 20,000 years.
•
An extra second was added between Dec. 31, 2008 and Jan.1,
2009 to allow for irregularities in the Earth’s rotation.
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Lunar Calendars
•
Lunar calendars follow the Moon’s cycle, which averages 29½
days per month.
•
Since the year would contain only 12 x 29.5 = 354 days, an
additional month was added usually every 3 years.
•
The Jewish calendar (now in the year 5767) is lunar, and is
synchronized with the solar calendar by following the 19 year
cycle, introduced by the Greek astronomer Meton in 432 BCE.
•
Easter has a partially lunar basis, being scheduled as the first
Sunday following the first full moon on or after March 21.
•
The Islamic calendar is purely lunar, so that 12 months
contain about 11 days fewer than a solar year.
•
That is why, for example, Islamic festivals, such as Ramadan
begin about 11 days earlier on each subsequent year.
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Phases of the Moon 1
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Sky at Sunset
The Moon’s position at sunset is shown for 14 evenings,
beginning at the new moon and ending at the full moon.
•
Note that west is to your right, which occurs if you are facing to
the south, so that the Sun sets to your right.
32
Sky at Sunrise
The Moon’s position at sunrise is shown for 14 evenings,
beginning at the full moon and ending at the new moon.
•
Note that west is to your right, which occurs if you are
facing to the south, so that the Sun sets to your right.
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PHASES OF THE MOON
PHASE
SHAPE
MOONRISE
WHEN VISIBLE AT
NIGHT
New moon
-
dawn
-
Waxing crescent
late morning
evening
1st quarter
noon
before midnight
Waxing gibbous
afternoon
until early morning
Full moon
dusk
All night
Waning gibbous
evening
from evening
3rd quarter
midnight
after midnight
Waning crescent
early morning
early morning
Solar and Lunar Eclipses 1
•
•
The plane in which the Moon orbits the Earth makes an
angle of 5.2o with plane of the ecliptic.
For an eclipse to occur, the Moon must be full or new at the
same time as its path crosses the ecliptic.
35
Solar and Lunar Eclipses
2
•
The line of nodes is a hypothetical line joining the two points
at which the Moon’s orbit crosses the ecliptic.
•
Eclipses occur when the line of nodes points towards the Sun.
36
The Eclipse Seasons
•
Eclipses are relatively rare, because for eclipses to
occur, the Moon must be full or new, just as it crosses
the ecliptic plane.
•
There are just two short periods in a year, known as the
eclipse seasons, when eclipses can occur, although
there is no guarantee of eclipses occurring during a
particular season.
•
Between 2 and 5 solar eclipses can occur in a year, and
a similar number of lunar eclipses. However, the total
number of eclipses in a year cannot exceed 7.
•
It was known to ancient astronomers that the basic
pattern of eclipses repeats every 18 years 11.3 days.
This repetition pattern is known as the Saros cycle.
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Solar Eclipses 1
•
•
A solar eclipse occurs when the Moon blocks some or all of
the Sun’s light, so that the Moon’s shadow falls on the Earth.
The umbra, the central region of the Moon’s shadow, is
surrounded by the penumbra .
•
Only in the umbra is the sunlight totally blocked, so that a total
solar eclipse or an annular solar eclipse occurs.
•
A total solar eclipse occurs when the Moon is relatively close
to the Earth, so that it appears large enough to totally blot out
the Sun, thus allowing the faint solar corona to be seen.
•
An annular solar eclipse appears as a thin ring encircling the
Moon’s disk when the Moon is too far from the Earth for it to
totally block out the Sun.
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Solar Eclipses 2
The umbra
forms a dark
spot, which is
the region of the
total or annular
eclipse.
39
Solar Eclipses 3
•
A total eclipse occurs when the
Moon is close enough to block out
the Sun’s surface, allowing its
outermost layer – the corona – to
be seen.
•
A partial eclipse is seen from the
shadow given by the Sun’s
penumbra.
•
An annular eclipse occurs when
the Moon is far enough away, so
that it cannot hide the Sun’s
surface completely.
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Total Solar Eclipse

Only during a total solar eclipse is the solar corona visible.
41
Annular Eclipse
42
Solar Eclipse Tracks 2000-2020

The width of the track depends both on the Earth’s latitude and
the distance of the Moon from the Earth during the eclipse.
Saros cycle
43
Lunar Eclipses 1
A lunar eclipse occurs when the Moon enters the Earth’s shadow.
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Lunar Eclipses 3
The Moon looks red during a total lunar eclipse for the same reason that
the Sun appears reddish at sunrise and sunset, and the sky appear blue.

Sunlight is composed of all the colors of the rainbow (red, orange, yellow,
green, blue, violet), and the Earth’s atmosphere preferentially scatters the
blue end of this spectrum of colors.

The scattered blue light gives the sky its color, while the missing blue end
of the spectrum makes the Sun appear yellow during the day and red at
sunrise and sunset, when the Sun’s rays take a longer path through the
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atmosphere to reach us.
Lunar Eclipses 2
•
•
The Moon appears red in a total lunar eclipse because of the
preferential scattering by the Earth’s atmosphere of the blue
end of the spectrum of colors in sunlight.
As a result, more of the Sun’s red light reaches the Moon.
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Lunar Eclipse Over Dallas
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