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Lecture 3, ASTA01
Chapter 2
User’s Guide to the Sky:
Patterns and Cycles
• In addition to the daily motion of the sky,
Earth’s rotation adds a second motion to
the sky that can be detected only over
• More than 2100 years ago, astronomer
Hipparchus compared a few of his star
positions with those made by other
astronomers nearly two centuries before
• He realized that the celestial poles and
equator were slowly moving relative to the
• Later astronomers understood that this
apparent motion is caused by a special motion
of Earth called precession.
• If you have ever played with a toy top or
gyroscope, you may recall that the axis of
such a rapidly spinning object sweeps
around relatively slowly in a circle.
• The weight of the top tends to
make it tip.
• This combines with its rapid
rotation to make its axis sweep
around slowly in precession
• Earth spins like a giant top, but it does not
spin upright relative to its orbit around the
• You can say either that Earth’s axis is tipped
23.5°from vertical or that Earth’s equator
is tipped 23.5°
relative to its orbit.
• “obliquity” 23.5°
• Earth’s large mass and rapid rotation keep
its axis of rotation pointed toward a spot
near Polaris (alpha Ursa Minoris).
• Its axis direction would not move if Earth were
a perfect sphere.
• However, Earth has a slight
bulge around its middle.
• The gravity of the Sun and
Moon pull on this bulge, tending
to twist Earth’s axis upright
relative to its orbit.
• The combination of these forces and
Earth’s rotation causes Earth’s axis to
precess in a slow circular sweep – taking
about 26 000 years
for one cycle.
• As the celestial poles and equator are
defined by Earth’s rotational axis,
precession moves these reference marks.
• You would notice no change at all from night
to night or year to year.
• Precise measurements, though, reveal their slow
apparent motion.
• Over centuries, precession has dramatic
• Egyptian records show that 4800 years ago
the north celestial pole
was near Thuban
(alpha Draconis).
• Now, the pole is
approaching Polaris
and will be closest to
it in about 2100.
• In about 12,000 years, the celestial pole
will have moved to the apparent vicinity of
the very bright star Vega (alpha Lyrae).
• The figure shows
the apparent path
followed by the north
celestial pole over
thousands of years.
The Cycle of the Sun
• Rotation is the turning of a body on its
• Revolution is the motion of a body around
a point outside the body.
• Earth rotates on its axis – and that produces
day and night.
• Earth also revolves around the Sun – and that
produces the yearly cycle.
The Annual Motion of the Sun
• Why don’t we see stars during day??
• Even in the daytime, the sky is actually
filled with stars.
• However, the glare of sunlight fills Earth’s
atmosphere with scattered light, and you can
only see the brilliant blue sky.
The Annual Motion of the Sun
• If the Sun were fainter and you could see
the stars in the daytime, you would notice
that the Sun appears to be moving slowly
eastward relative to the background of the
distant stars.
• This apparent motion is caused by the real
orbital motion of Earth around the Sun.
The Annual Motion of the Sun
• In January, you would see the Sun in front of the constellation
Sagittarius. By March, it is in front of Aquarius.
The Annual Motion of the Sun
• Note that your viewing angle in the figure makes the
Earth’s orbit seem very elliptical when it is really
almost a perfect circle.
The Annual Motion of the Sun
• Through the year, the Sun moves
eastward among the stars following a line
called the ecliptic – the apparent path of
the Sun among the stars.
• If the sky were a great screen, the ecliptic
would be the shadow cast by Earth’s orbit.
• In other words, you can call the ecliptic the
projection of Earth’s orbit on the celestial sphere.
The Annual Motion of the Sun
• Earth circles the Sun in 365.26 days and,
consequently, the Sun appears to go once
around the sky in the same period.
• You don’t notice this motion because you
cannot see the stars in the daytime.
• However, the motion of the Sun caused by a real
motion of Earth has an important consequence
that you do notice – the seasons.
The Seasons
• The seasons are
caused by the
revolution of Earth
around the Sun
combined with a
simple fact you
have already
• Earth’s equator is
23.5°relative to its
The Seasons
• There are two
important principles
to note about the
cycle of seasons.
The Seasons
• The seasons are not caused by variation in the distance
between Earth and the Sun.
• Earth’s orbit is nearly circular, so it is always about the same
distance from the Sun. In fact, the sun is closer in the winter!
The Seasons
• The seasons are caused by changes in the amount
of solar energy that Earth’s northern and southern
hemispheres receive at different times of the year –
resulting from the tip of the Earth’s equator and axis
relative to its orbit.
The Seasons
• The seasons are so important as a cycle of
growth and harvest that cultures around
the world have attached great significance
to the ecliptic.
• It marks the centre line of the zodiac (‘circle of
• Also, the motion of the Sun, Moon, and the five
visible planets (Mercury, Venus, Mars, Jupiter, and
Saturn) are the basis of the ancient superstition of
• However, the signs of the zodiac are no longer
important in astronomy.
The Planets
• You can look for the planets along the
ecliptic appearing like very bright stars.
• Mars looks quite orange in colour.
• Saturn and Jupiter are silver and usually
• Venus is the brightest. Sometimes it’s so
close to the sun (in degrees) that it’s hidden in
sun’s glare.
• Uranus and Neptune cannot be seen by
naked eye. Pluto & asteroids are hard to see
even with moderate-sized telescopes.
The Planets
• As Venus and Mercury
orbit inside Earth’s orbit,
they never get far from
the Sun and are visible in
the west after sunset or
in the east before
• Venus can be very bright,
but Mercury is difficult to
see near the horizon.
The Planets
• By tradition, any planet in the sunset sky is
called an evening star.
• Any planet in the dawn sky is called a
morning star.
• Both names are misnomers! They’re not
The Planets
• Venus, which can become as bright as
magnitude -4.7
• That’s almost 100 times brighter than
• As Venus moves around its orbit, it can
dominate the western sky each evening for
many weeks.
• Eventually, its orbit appears to carry it back
toward the Sun as seen from Earth, and it is
lost in the haze near the horizon.
The Planets
• A few weeks later, you can
see Venus reappear in the
dawn sky as a brilliant
morning star. Example:
September 2012.
• Months later, it will switch
back to being an evening
The Cycles of the Moon
• The Moon orbits eastwards around Earth
once a month.
The Cycles of the Moon
• Starting this evening, look for the Moon in
the sky.
• If it is a cloudy night or if the Moon is in the
wrong part of its orbit, you may not see it.
• Keep trying on successive evenings.
• Within a week or two, you will see the Moon.
• Then, watch for the Moon on following
• You will see it move along its orbit around Earth
and cycling through its phases as it has done for
billions of years.
The Motion of the Moon
• If you watch the Moon night after night, you will
notice two things about its motion.
• First, you will see it moving relative to the
background of stars. A related thing is that the
Moon rises and sets about 50 minutes later every
day. Every day it’s seen to the east of the previous
• Second, you will notice that the markings on its
face don’t change.
• These two observations will help you understand the
motion of the Moon and the origin of the Moon's phases.
The Motion of the Moon
• The Moon moves continuously & rapidly
among the constellations.
• If you watch the Moon for just an hour, you
can see it move eastward against the
background of stars by slightly more than its
own apparent diameter.
• Each night when you look at the Moon, you will
see it is roughly half the width of a zodiac
constellation—about 13 degrees—to the east of its
location the night before.
• This movement is the result of the motion of the Moon
along its orbit around Earth.
The Cycle of Moon Phases
• The changing shape of the illuminated part
of the Moon as it orbits Earth is one of the
most easily observed phenomena in
The Cycle of Moon Phases
• There are three important points to notice
about the phases of the Moon.
The Cycle of Moon Phases
• First, the Moon always keeps the same side
facing Earth, and you never see the far side of
the Moon. Spin and orbit are in 1:1 period ratio.
• ‘The man in the moon’ (some cultures see ‘the rabbit in the moon’
instead) is produced by familiar features on the Moon's near
Q: Is there a “dark side of the Moon” ??
The Cycle of Moon Phases
• Second, the changing shape of the Moon as it
passes through its cycle of phases is produced
by sunlight illuminating different parts of the side
of the Moon you can see.
The Cycle of Moon Phases - day 2
Gray moon
The Cycle of Moon Phases – day 5
The Cycle of Moon Phases – day 7
The Cycle of Moon Phases – day 14.5
The Cycle of Moon Phases –
siderial (27.3 days) & synodic (29.5 days) periods
The Cycle of Moon Phases –
siderial (27.3 days) & synodic (29.5 days) periods
Siderial = with respect to stars
Synodic = meeting other bodies in the same configuration
The Cycle of Moon Phases – day 27
Watch the Youtube video with accurate representation of
moon phases during one full year:
This is an animation of what you’d see over 12 months
(in 2012) if you were able to watch the Moon all the
time. It’s based on detailed maps of the Moon.
The Moon seems to wobble left and right – that’s a true
physical effect called libration; it’s due to its orbit’s
ellipticity and thus uneven motion of the Moon around
It also tilts up and down – it’s not a physical effect, rather
the Moon is on an orbit slightly inclined to the Earth’s equator
so over one month we do see it a bit from below (south pole
better than north pole) and then from above (more of a north