Tools

Sidereal Time Calculator

What This Calculator Tells You

Sidereal time is "star time" - it measures Earth's rotation relative to the stars rather than the Sun. Local Sidereal Time (LST) tells you which right ascension is currently crossing your meridian (due south in the northern hemisphere). Objects near this RA are at their highest point and best for observation.

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Understanding Sidereal Time

  • 24 sidereal hours ≈ 23h 56m 4s - A sidereal day is about 4 minutes shorter than a solar day
  • LST = RA on meridian - Whatever right ascension equals your LST is currently transiting (at its highest)
  • Same star rises 4 min earlier each night - This is why constellations shift with the seasons

Using Sidereal Time

Planning Observations

Find objects with RA close to your current LST - they're near the meridian and highest in the sky. Objects 6h away from LST are rising or setting.

Telescope Alignment

Many computerized mounts need LST for initial alignment. Some mounts have built-in clocks, but manual entry is sometimes required.

Hour Angle

Hour Angle = LST - Object's RA. A negative hour angle means the object hasn't reached the meridian yet; positive means it's past.

Setting Circles

Manual equatorial mounts use setting circles with RA. Set your RA circle to match LST on a known star, then dial in any other object.

Rules of Thumb

  • LST advances 1 hour in ~1 hour: Actually about 59 minutes 50 seconds
  • 2 hours of RA = 30°: Each hour of RA equals 15° of sky
  • Objects visible for ~12 hours: From RA 6h before LST to 6h after
  • Same LST at same clock time = same season: LST depends on date and local time

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