Download The Celestial Sphere

The Celestial Sphere
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The Celestial Sphere
•  Geocentrially speaking, the
Earth sits inside a celestial
sphere. Fixed on the sphere
are stars.
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Constellations
•  Constellations are patterns of
stars that the eye picks out.
The stars are usually not near
each other; they just lie in
the same direction.
•  Historically, the brightest
stars are stars of the 1st
magnitude.
The next
brightest are stars of the 2nd
magnitude.
The faintest
stars one can see by eye are
6th magnitude. (The biggest
telescopes can reach m = 26
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or so.)
Orion, the Hunter
Cygnus, the Swan
Scorpius, the Scorpion
Antlia, the Air Pump
Leo, the Lion
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Puppis, the Poop Deck
Constellations
Andromeda (Princess)
Circinus (Compasses)
Lacerta (Lizard)
Piscis Austrinus (Southern Fish)
Antlia (Air Pump)
Columba (Dove)
Leo (Lion)
Puppis (Poop Deck)
Apus (Bird of Paradise)
Coma Berenices (Bernice’s Hair)
Leo Minor (Little Lion)
Pyxis (Compass Box)
Aquarius (Water Bearer)
Corona Australis (Southern Crown)
Lepus (Hare)
Reticulum (Net)
Aquila (Eagle)
Corona Borealis (Northern Crown)
Libra (Scales)
Sagitta (Arrow)
Ara (Altar)
Corvus (Crow)
Lupus (Wolf)
Sagittarius (Archer)
Aries (Ram)
Crater (Cup)
Lynx (Lynx)
Scorpius (Scorpion)
Auriga (Charioteer)
Crux (Southern Cross)
Lyra (Lyre)
Sculptor (Sculptor)
Boötes (Herdsman)
Cygnus (Swan)
Mensa (Table)
Scutum (Shield)
Caelum (Engraving Tool)
Delphinus (Dolpin)
Microscopium (Microscope)
Serpens (Serpent)
Camelopardalis (Giraffe)
Dorado (Swordfish)
Monoceros (Unicorn)
Sextans (Sextant)
Cancer (Crab)
Draco (Dragon)
Musca (Fly)
Taurus (Bull)
Canis Venatici (Hunting Dogs)
Equuleus (Little Horse)
Norma (Carpenter’s Square)
Telescopium (Telescope)
Canis Major (Big Dog)
Eridanus (River)
Octans (Octant)
Triangulum (Triangle)
Canis Minor (Little Dog)
Fornax (Furnace)
Ophiuchus (Serpent Bearer)
Triangulum Australe (Southern
Triangle)
Capricornus (Sea Goat)
Gemini (Twins)
Orion (Hunter)
Tucana (Toucan)
Carina (Keel)
Grus (Crane)
Pavo (Peacock)
Ursa Major (Big Bear)
Cassiopeia (Queen)
Hercules (Hercules)
Pegasus (Winged Horse)
Ursa Minor (Little Bear)
Centarus (Centaur)
Horologium (Clock)
Perseus (Hero)
Vela (Sails)
Cepheus (King)
Hydra (Water Serpent)
Phoenix (Phoenix)
Virgo (Maiden)
Cetus (Whale)
Hydrus (Water Snake)
Pictor (Painter’s Easel)
Volans (Flying Fish)
Chamaeleon (Chameleon)
Indus (Indian)
Pisces (Fishes)
Vulpecula (Fox)
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Diurnal Motion
•  Look at the diagram and
imagine yourself standing
somewhere on Earth. The
Earth is spinning. What
would you see over the
course of a day when you
look up at the celestial
sphere???
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Diurnal Motion
•  The Earth’s axis of rotation
also defines the key places
on the celestial sphere.
•  The celestial North pole is
over the Earth’s north pole.
•  The celestial equator is the
extension of the Earth’s
equator.
•  The meridian divides east
from west on the sky.
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Diurnal Motion
•  Each day, the Earth rotates
once (west-to-east) on its
axis. This causes us to face
different directions and see
different stars. The stars’
daily (diurnal) motion
reflects the Earth’s spin.
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Risings and Settings
The spin of the Earth causes the stars to appear to rotate
about the celestial pole. Some stars are circumpolar and
never set, while others dip below the horizon. Which stars
are which depends on where you are on Earth.
The 2nd magnitude star Polaris happens to be very near the
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North celestial pole.
The Yearly Motion
In addition to rotating, the Earth also revolves about the Sun.
As the Earth revolves
the Sun is projected in
front of different
constellations at
different times of year.
The path the Sun takes
across heavens is called
the ecliptic.
The
constellations which the
Sun passes through are
zodiac constellations.
Because the Sun is bright, we can only see some
constellations at certain times of year.
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The Seasons
Since the plane of the ecliptic is tilted 23.5° with respect
to the celestial equator, we have seasons.
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The Motion of the Sun
From the Geocentric point of view, the Sun moves from the
northern part to the southern part of the sky with the seasons.
When the Sun is furthest north (south), it’s the summer (winter)
solstice. When the Sun crosses the celestial equator, it’s the
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vernal (or autumnal) equinox.
The Day
Because the Earth revolves about the Sun, the word “day” has two
definitions.
A sidereal day is one
rotation with respect
to the stars.
A solar day is one
rotation with respect
to the Sun.
Sidereal and solar
days differ by about
4 minutes.
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Precession
In addition to its rotation and revolution, the Earth’s axis also
precesses (wobbles) like a top. The angle between the ecliptic
and the equator remains at 23.5°, but the direction changes.
The period of this precession of the equinoxes is about 26,000
years.
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Because of precession:
•  Polaris won’t always
be the North Star
•  A sidereal year is not
quite a Julian year.
•  Orion won’t always be
a winter constellation
•  Horoscopes are all
wrong (since those
tables don’t include
precession.)
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The Lunar Period
The Moon moves westto-east in the sky (like the
Sun), and takes about a
month to circle the Earth
(hence the word month).
But once again, there’s a
difference between the
Moon’s sidereal period
with respect to the stars
(27 days), and the
synodic period with
respect to the Sun (29
days).
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Phases of the
Moon
The Moon reflects light
from the Sun, so its
phase tells you the
relative position of the
Sun.
You can therefore tell
time from the Moon!
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Phases of the
Moon
We tell time by the Sun: it
rises around 6 a.m., is
high in the sky at noon,
and sets around 6 p.m.
The phase of the Moon
tells you the angle
between the Moon and
Sun. So when you see the
Moon, you know where
the Sun is!
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Phases of the
Moon
Full Moon occurs when
the Moon is opposite in
the sky from the Sun.
This is called opposition.
New Moon occurs when
the Moon is in the same
direction as the Sun, i.e.,
in conjunction.
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The Moon’s
Shadow
The Moon’s orbital
plane is tilted by 5.2
degrees from the
ecliptic plane.
Hence, ½ the time,
the Moon is slightly
north of the ecliptic
(and ½ the time, it
is south of the
ecliptic).
The
shadow of one body
very rarely falls on
the other.
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Lunar Eclipses
If the Moon crosses the ecliptic plane while exactly opposite the Sun, it
will fall in the Earth’s shadow. This is a lunar eclipse. This happens
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roughly twice a year.
Solar Eclipses
If the Moon crosses the
ecliptic plane while
exactly in front of the
Sun, a tiny piece of the
Earth will fall in the
Moon’s shadow. This is
a solar eclipse. Since the
Moon and Sun appear to
be the same size in the
sky, the alignment of a
total solar eclipse must be
perfect.
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Total Solar Eclipse
You only get to see this outer atmosphere (corona) of the Sun if
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the Sun’s entire body is blocked out.
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Partial Solar Eclipse
If you’re on the edge of an eclipse path and only a slice of the Sun is
blocked out, it’s called a partial eclipse. These are not very exciting,
since the uneclipsed part of the Sun is still extremely bright.
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Annular Eclipses
Because the Moon’s orbit
about the Earth is not perfectly
circular, it is sometimes too far
away to block out the whole
Sun, even when perfectly
aligned.
When this happens, you get an
annular eclipse. It is rarer than
a total solar eclipse, but, like
other partial eclipses, they are,
in general, pretty boring.
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Solar Eclipse Paths through 2025
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