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Constellations
• What is a
constellation?
• A group of stars that
seems to form a
picture
• What are circumpolar
constellations?
• What are seasonal
constellations?
• Constellations that
appear to move around
the pole star, Polaris
• They can be seen all
year e.g., Ursa Major,
Cepheus
• Constellations that cannot
be seen all year.
- e.g. Orion, Leo, Pegasus
• What are zodiac
constellations?
• Seasonal constellations
that are associated with the
astrological signs of the
zodiac
e.g. Aries, Taurus, Cancer
What’s your sign?
When we set the star-finders to our birthdays, we discovered we could
not see “our” zodiac constellation on the night of our birthday,,,,
To understand why, we need to
look at Earth in space….
Remember that Earth is tilted on its axis of rotation.
So if you look at Earth straight up and
down, it would seem like the solar
system is tilted instead:
The “celestial sphere”
That’s why here
on Earth, the sun
and planets
appear to go
around the Earth
along a slanted
pathway that
mirrors the plane
of the solar
system.
We call this path
the ecliptic.
The celestial equator is an imaginary
line mirroring Earth’s equator.
Notice where the sun is
in relation to the
celestial equator during
the different seasons.
What happens on the
equinoxes?
Here is a view from Earth looking into the sky: The green line is the ecliptic, the pathwa
the sun appears to follow. The red line is the celestial equator. What day is this?
The ancient astrologers chose constellations that are found along the
ecliptic as the constellations of the Zodiac.
Try to picture the plane of the solar system with the stars and constellations
far, far away in all directions. The ones lined up along the plane of the solar
system will look like they are on the sun’s path – the ecliptic.
Ancient astrologer chose constellations along the ecliptic to represent the
signs of the zodiac. Signs are assigned to birthdates according to when the
sun appears to be “in” the particular constellation.
If the sun is “in” your constellation on your birthday, that means it won’t be
out at night, so you will not see “your” zodiac constellation on your birthday!
At least it’s supposed to be – but there
is a complication!
We learned that Earth has 2 major motions:
Rotation and Revolution.
But there is a THIRD motion……
It’s called “precession” and it means that
just like a spinning top, Earth wobbles
a bit as it spins …
A top wobbles because Earth’s gravity is pulling on it.
Earth “wobbles” because the
moon’s gravity is pulling on it.
Unbeknownst to the ancient astrologers, the Earth continually wobbles
around its axis in a 25,800-year cycle.
That means, Polaris isn’t always our
pole star.
It also means, that since the time people
made up the star signs, the earth has
wobbled enough that the sun really isn’t in
“your” sign on your birthday.
The sun is actually one sign behind – so when the calendar says it’s Aries, the
sun is actually in Pisces, and so forth…
And here’s another complication, the ancient
Babylonians actually had 13 signs!
They also included Ophiucus!
Which would further change the zodiac sign most people think they have if
we decided to bring this sign back:
Not sure if
whoever
made this
accounted
for
precession
as well –
you can
check if you
use a Star
finder…..
(or an
iPhone
app…)
Horoscopes and zodiac signs
are fun, but they don’t make any
sense scientifically!
YOU
are the master of
your fate,
not the stars!!
See of you can find the ecliptic and celestial equator
on this diagram: