or an Astronomical Sky Clock
This sky clock is designed to show you what's happening in the sky over your head right now. Specifically,
it shows you where to find the Sun, the Moon, the five planets visible to the naked eye, and four bright stars that happen to travel near them.
Here is the basic grade-school illustration of the Solar System, the one that shows the planets
rolling around the Sun like peas on a plate. Actually, this is a stripped-down view where we're
looking only at the Sun, the Earth, the Moon, Venus, and Mars. Viewed from far above the Sun,
it looks something like this.
But forget about the diagram for a second and think about what your experience of viewing planets
is really like. We don't have this God's eye view of the Solar System with big circles showing
the orbit of each planet. Instead, we see little bright spots on a dark background somewhere "up there."
Some spots are bigger and some are smaller, but how far away are they? It's hard to say. The Sun and
the Moon appear to be the same size despite the fact that the Sun is nearly 400 times farther away than the Moon.
In some basic sense, everything in the sky appears to be painted on a celestial dome that
is effectively the same distance from your nose.
Since all the planets are confined to the same plane, it means that, from our point of view
here on Earth, the Sun and all the planets (and the Moon too, as it happens) appear to move along the same path through the sky. This path, known as the ecliptic, passes through a series of twelve constellations collectively known as the zodiac. The zodiac thus acts as a sort of backdrop that the planets appear to move across.
As a final complication of our picture, we can only see one half of the sky at any given time, since we're standing on a planet that blocks the rest of our view. Since the Earth turns, the sky appears to move around us. Just as the Sun rises in the east and sets in the west, so too do each of the planets, in their turn, rise in the east and set in the west.
The Sky Clock is designed to show you what's happening on the ecliptic right now. The Sun and Moon have been moved onto their own tracks for clarity, but the only variable being accurately represented is the angular position along the ecliptic. Everything in the top half of the diagram is in the sky right now. Everything in the bottom half, like the sun at night, is out of view beneath your feet.
Shown below is an animation of how the Sky Clock changes during a single day. Notice how the sky is light blue
when the Sun is above the horizon and dark blue at night. The image of the Sun
effectively acts like the hand of a 24 hour clock. Notice how everything in the sky
moves more or less together, rising in the east and setting in the west.
Next is an animation of a month. The snapshots have been taken at midnight of each day,
so the Sun appears to be motionless at the bottom of the image. Now we see how the Moon,
planets, and stars move relative to the Sun. The Moon moves especially quickly, completing a
counterclockwise circuit in one month. The small "Moon Clock" inset on the upper right
of the diagram indicates the phase of the Moon. The stars move relative to the Sun in a clockwise
direction. This signifies the changing backdrop of stars we see as the Earth circles the
Sun. For the month pictured, the Sun is moving through Taurus as Libra passes through the zenith
Finally, we see an animation of the Sky Clock throughout an entire year. Again, the
snapshots occur at midnight, so the Sun appears to be motionless. There are a number of
things to notice here. First, the Sun moves through the entire zodiac in one year,
this being the definition of a year. We also observe that Mercury and Venus never stray
very far away from the Sun. Since they are both inside our orbit, their elongation
from the Sun is strictly limited. The two yellow radial lines emanating from the center
of the diagram indicate the eastern and western horizons at sunrise and sunset respectively.
Since this animation was made for a location at a nothern latitude, the length of the day
changes with the seasons, and the yellow lines flap up and down.
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