Notice that, from our vantage point on Earth, we see the Sun appear to move steadily on an eastward path through the constellations of the zodiac.
Remember that this path is called the ecliptic, and this is why the ecliptic is marked on the celestial sphere as we first saw in Figure 2. Use Figure 2. What constellation of the zodiac will be on your meridian at midnight tonight?
Name at least four other zodiac constellations that will be visible in your sky at midnight. They all pretty much circle in the same direction as they revolve in their wide, lengthy galactic orbits, but each star's path is its own, like the orbits of the planets around the Sun.
Each star's orbit may be inclined tilted with respect to others, and their velocities depend in large part on their distance from the Milky Way center.
It's not unlike the cars going in one direction on a freeway: they all speed along in the same direction, but with slight differences in speed and occasional lane changes that result in gradual changes in position and sometimes not so gradual; I'm talking to you, driver of the red sports car!
Those slow relative changes in position give each star in our sky a particular " proper motion "—a change in angular position. Hold up your pinky finger at arm's length: the width of your pinky, expressed as an angle, is about one degree, give or take. One degree is 3, arcseconds, or 3,, milli-arcseconds. Now for the big reveal: the star with the fastest proper motion in our skies at this time : Barnard's Star , a small, low-mass red dwarf about 6 light years away in the constellation Ophiuchus, the Snake-holder, just above Scorpio kind of a creepy section of the sky.
Have you ever wondered why most star patterns are associated with specific seasons of the year? Just why, for instance, can evening skywatchers in the Northern Hemisphere enjoy Orion the Hunter only during the cold wintry months? During balmy summer evenings it is not Orion, but the stars of Scorpius, the Scorpion, that dominate the southern sky. Spring evenings provide us with a view of the Sickle of Leo, the Lion.
Were we to watch the sky on any night from dusk to dawn we would notice certain stars rising from above the eastern horizon in the evening hours. They would sweep across the sky during the night , finally setting beneath the western horizon by dawn. No big deal here, since, after all, the sun does the same thing during the daylight hours. Those stars that were low over the western horizon during the early evening hours would, within a matter of a few weeks, disappear entirely from our view, their places taken up by groups that a few weeks earlier were previously higher in the sky at sundown.
In fact, it would seem that with the passage of time, all the stars gradually shift westward while new stars move up from the eastern horizon to take their place. Are the stars in a constellation near each other? Not necessarily. Each constellation is a collection of stars that are distributed in space in three dimensions — the stars are all different distances from Earth.
The stars in a constellation appear to be in the same plane because we are viewing them from very, very, far away. Stars vary greatly in size, distance from Earth, and temperature. Dimmer stars may be smaller, farther away, or cooler than brighter stars.
By the same token, the brightest stars are not necessarily the closest. Of the stars in Cygnus, the swan, the faintest star is the closest and the brightest star is the farthest! How are constellations named? Most of the constellation names we know came from the ancient Middle Eastern, Greek, and Roman cultures. They identified clusters of stars as gods, goddesses, animals, and objects of their stories.
It is important to understand that these were not the only cultures populating the night sky with characters important to their lives. Cultures all over the world and throughout time — Native American, Asian, and African — have made pictures with those same stars.
In some cases the constellations may have had ceremonial or religious significance. In other cases, the star groupings helped to mark the passage of time between planting and harvesting. There are 38 modern constellations. In the International Astronomical Union officially listed 88 modern and ancient constellations one of the ancient constellations was divided into 3 parts and drew a boundary around each. The boundary edges meet, dividing the imaginary sphere — the celestial sphere — surrounding Earth into 88 pieces.
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