Chapter 1: Charting the Heavens - Saratoga High School

Chapter 1:
Charting the Heavens
1.3: Constellations
In this chapter we explore how the universe appears from our
vantage point on Earth.
People have been observing the sky for thousands of years. Many
groups of people through history have named various groupings of
stars as constellations. The stars in a constellation do not usually
have any association with one another except that they make a
pattern as seen from Earth.
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1.3: Constellations
There are 88 constellations in the sky; 44 of them were
named in ancient times (Mesopotamian, Babylonian,
Egyptian, Greek.) They represent such things as animals,
heroes, and goddesses. These 44 don’t cover the whole
sky.
They miss: 1) regions of sky with no bright stars
– 2) the far southern sky
Modern astronomers created 44 more
constellations to fill in the gaps between the
ancient ones, so that every part of the sky falls
within the boundaries of a constellation. This is
because many astronomical objects are named or
located based on the constellation in which they
are found.
1.3: Constellations
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1.3: Constellations
One example of this is stars. Stars are named in
two ways: individual names (for the brightest stars
only) and based on constellation. Within a
constellation, stars are named with greek letters,
beginning with alpha (α) as the brightest, beta (β)
as the second brightest, and so on. So the
brightest star in the constellation Taurus, named
Aldebaran, is also known as α Tauri. These
designations were made in ancient times for the
original 44 constellations, so they aren’t always
completely accurate.
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Brightness
The brightness of objects is measured using
apparent visual magnitude. The magnitude
scale is an inverse scale: lower numbers mean
brighter objects. It describes how bright the
stars look to human eyes (which does not
necessarily relate to how much light they put
out). Amateur astronomers use this scale more
than professional astronomers. Astronomers
use an Intensity scale which measures the
amount of light energy that hits 1 square meter
in 1 second.
Object
Magnitude
Sun
Moon (full)
Sirius ( the brightest star in the
sky)
Dimmest star visible from
Saratoga
Dimmest star visible to the
Hubble Space Telescope
-26.5
-12.5
-1.4
+5 to +6
+28
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The Celestial Sphere
• Our model of the sky as seen from Earth is called the
celestial sphere – a large imaginary sphere that surrounds
the planet to which all celestial bodies seem to be attached.
We only see half of this sphere at any given time, the rest
is below the horizon, where sky meets Earth.
• On the celestial sphere we have reference marks to guide
us in the sky. There are north & south celestial poles –
points in the sky directly above the north & south poles of
Earth. These points appear motionless in the sky, as the
celestial sphere rotates around them.
There is a celestial equator, the imaginary line directly
above the Earth’s equator.
The Celestial Sphere
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Angular Measure
• To measure distances in the sky we use angles, measured in
degrees arc minutes & arc seconds.
In 1 o there are 60’ and in 1’ there are 60”
These angle measurements are used to describe separations,
diameters, and elevations of objects in the sky.
If you extend your arm, the width of your index finger ≈ 1 o
while the width of your fist ≈ 10 o
Precession
• We generally think of the celestial sphere as fixed,
but it isn’t. The celestial poles slowly move across
the sky, as if the Earth is wobbling like a top. This
is called precession. The celestial poles make a
circle through the sky, taking 26,000 years to
complete one cycle.
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Seasonal Changes
• There is one more important line we must draw
on the celestial sphere: the ecliptic. This is the
path that the sun appears to follows through the
sky. The sun is in a slightly different location in
the sky each day due to the Earth’s motion around
the sun. The tilt of the Earth’s axis causes the
ecliptic to curve through the sky rather than
follow a straight line.
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Seasonal Changes
• There are 4 special points on the ecliptic that relate
to our terrestrial seasons:
• vernal equinox – where the sun crosses the celestial
equator moving northward – the start of spring
• summer solstice – the sun’s northernmost point in
the sky – the start of summer
• autumnal equinox – where the sun crosses the
celestial equator moving southward – the start of
fall
• winter solstice – the sun’s southernmost point in the
sky – the start of winter
• The angle of incidence of the sun’s rays is what
gives us seasons
Summer in the Northern
Hemisphere
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Winter in the Northern
Hemisphere
Milankovitch Hypothesis
• The Earth’s closest approach to the sun, called perihelion, is
actually in January; we are furthest from the sun, at aphelion,
in July.
• The change in distance from Earth to sun does not give us
seasons – the Earth’s orbit is too close to a perfect circle for
that. It is the fact that we get more direct rays from the sun in
the summer that makes it a warmer season than winter.
• The changes in the shape of Earth’s orbit, in precession, and in
the tilt of Earth’s axis, have led to the Milankovitch
hypothesis – that these variations may combine to cause
global climate changes such as ice ages.
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The planets and the ecliptic
• The planets also
remain close to
the ecliptic as
they travel
across the
celestial sphere,
because they all
orbit in roughly
the same plane
as the Earth’s
orbit.
• Photo: Saturn,
Venus, Jupiter,
Mars on Ecliptic
Signs of the Zodiac
• Ancient astrologers defined the zodiac, a
band of constellations around the ecliptic.
Due to precession, however, the sign you
were born under probably isn’t the sign that
the sun was actually in when you were born.
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