Sidereal vs. Solar Time

Try the following exercise to help get a better feel for the difference between solar and sidereal time. Choose a time of day that you are typically free, this is important because you will need to repeat this procedure at the same time of day on different weeks.

Procedure

  1. To load the local sidereal time page, go to the Home Page of the Time Services Department at the U.S. Naval Observatory. Follow the "What Time Is It?" link, and from that page select "Compute Local Apparent Sidereal Time".
  2. Enter Houston, TX as the city and click the Compute button.
  3. Write down the date, mean solar time (e.g. 11:32 am, CST), and the local apparent sidereal time the page displays. Make sure to not loose the piece of paper that you write this down on, since you will want to refer to it for the next part.
  4. Now make a prediction, if you tried the same procedure one week later, what would the relationship between the local sidereal time and the solar time (e.g. CST or CDT) be? for example, if you loaded the page at 11:32 am CST, and the LST was 22:37, would you expect the LST one week later at 11:32 CST to be 22:37, or earlier or later than that?) If you expect the LST to be earlier or later, by about how many minutes earlier or later would you expect the LST to be, compared to 22:37 (or whatever LST you actually found when you loaded the page).
  5. One week later at approximately the same time of day, repeat the above procedure. Does the result for the LST match your expectations?