Skynow Starchart clock

• Click here to view the starchart clock.
• The starchart clock shows the current appearance of the sky (except that stars are visible in the daytime).
• The circle represents the horizon, with the four directions north, south, east, and west marked. The center of the circle represents the zenith (the point directly overhead).
• The scale around the circle gives the standard time, depending on the position of the sun. When the sun is above the horizon, a yellow clock hand moves along the scale; when the sun is below the horizon the clock hand turns dark blue.
• If daylight savings time is in effect, the scale will show daylight savings time.
• The phase and position of the moon are shown, along with the planets Venus, Mars, Jupiter, and Saturn.

• The clock hand moves counterclockwise. This is because the sun appears to move left to right to a northern hemisphere observer looking at the sun (which will be mostly to the south). Our normal definition of clockwise comes from the fact that the shadow of a sundial moves clockwise--but the shadow moves in the opposite direction as the actual sun.
• The green month abbreviations indicate the approximate position of the sun on the first of each month. These markers trace out an arc representing the ecliptic -- the apparent path of the sun in the sky.
• The gray numbers that follow an arc from due west to due east indicate the scale of right ascension. These numbers follow the arc of the celestial equator. The number along the meridian (the line from the zenith to due south) indicates the current sidereal time. Sidereal time is an indication of the position of the sky. The sky appears to make one revolution is 23 hours, 56 minutes; this period is called a sidereal day. 24 hours of sidereal time equal one sidereal day, so an hour of sidereal time is slightly different from an hour of ordinary (solar) time. The difference is caused by the Earth's orbit about the sun. During the course of one day the Earth rotates once, and moves about 1/365 of the way about its orbit. This means that the stars visible at midnight tonight are in a slightly different position than they were last night at midnight.
• Only selected stars are shown, and positions are approximate. Use a professional planetarium program for more detailed star positions.
• The time on the starchart clock depends on the time on your computer, so you need to make sure that time is set accurately.
• The actual position of stars will be slightly different if you are not at the central longitude for your time zone.
• The starchart is set for an observer at latitude 47 degrees north, but it will be useful for all mid-northern latitudes.
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