Sidereal Time for Civil Time

What Part of the Celestial Sphere is Visible this Time of Night?

A chart of civil time (time on your watch) versus sidereal time (ST -- time by the stars) is handy for planning a nightly observing/imaging session -- especially at odd hours when you are not sure what part of the sky is visible. Due to the relationship between sidereal time and right ascension (RA) coordinate, an object is on the meridian when the ST is equal to its RA position. The meridian, where the light passes through the least atmosphere, is the best place for observing and imaging objects.

All that is needed to create a chart of ST during a civil day is the date, observer's longitude and time zone correction. This correction is the number of hours the site is ahead or behind Universal Time -- commonly though of as Greenwich Mean Time or Zulu time. Because some places do not observe summer time or have intermediate zone corrections (30-minutes) this must be manually entered and not calculated. Note that in the Americas this will be a negative numbers of hours (example: EST = -5, PST = -8 and in summer: EDT = -4, PDT = -7).

Input Data for Chart

Data input here is displayed in a table above the chart.
Observer's Longitude (positive west):
Local time difference from Universal Time in hours:
and the UT Date: ; ;

Chart Computations

Chart and Explanation

How this Chart is Setup

  • Numbers across the bottom axis are hours of the day that correspond to the time on your watch (not UT!) in 24-hour format. Midnight at start and end of day is at left and right vertical sides of the graph. Noon is marked by a dashed red line.
  • Numbers on the vertical axis are sidereal time.
  • The blue data line indicates the sidereal and civil time at any instant of the day that the chart is generated for.

How to Read the Chart

  • Find the watch time of interest along the horizontal row of numbers and then go up until you meet the data line. Trace this point to the vertical scale on the left to find the sidereal time.
  • Alternately in Mathcad you can select the Trace option (try right-clicking on the graph), select the Track Data Points check box, left-click in the graph and drag the pointer left and right to see the watch time of interest in the X-Value box and the Sidereal Time in the Y-Value box.
  • The blue data line is vertical at zero hours of sidereal time -- when the equinox is overhead.

Astro Utilities Electronic Book Copyright 1999 Pietro Carboni. All rights reserved.


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