Sidereal: user's manual

Sidereal is a very simple and small program intended to display continuously your local sidereal time. In addition to sidereal time, it can also display local time and date, UTC time and date, mean and real solar time, and the modified julian date. Emphasis is given to accuracy. Sidereal is free software.


Unpack the ZIP archive, look for the file named sidereal.prc and install it on your PDA like you would install any other Palm OS application. The archive contains also the manual (sidereal.html, this file), the user license (COPYING, this is the GNU GPL for free software) and a directory called src with the sources.


Simple: just watch the time fly ;-). You can select what item you want to display in each of the four slots using the associated popup list.

Time items

The main screen can display any four items among:


This is just your local time, as known by the system. It's the same that would be displayed by any other clock on the same device, except that it's always displayed using the international notation.


Your real solar time. Also called apparent solar time, local apparent time, or simply LAT. This is what an uncorrected sundial would display. The Sun is due south at noon LAT.

Mean solar

Your mean solar time, local mean time, or LMT. It's the solar time corrected for the unsteadiness of the apparent movement of the Sun. This correction is called equation of time. If you were in the zero meridian, mean solar time would just be universal time (UT1, formerly called Greenwich mean time). Therefore, LMT is just universal time corrected for your longitude.


Your sidereal time, local mean sidereal time or LMST. This is the right ascension of your local zenith, and also the hour angle of the vernal equinox.

Sidereal computes Greenwich mean sidereal time using a linearization of Aoki's formula [S. Aoki et al., Astron. Astrophys. 105, 359 (1982)]. It then corrects for your longitude to get your local sidereal time. Look at the comments in the file calc.c for more details.

Local date

The date associated to your local time. It's the same date you see in the date book.


Coordinated universal time, or just universal time. That's what most people mistakenly call GMT. It's you local time corrected for your time zone.

There is another version of universal time called UT1. UT1 is tied to the rotation of the globe, including the irregularities of this rotation. It's the basis for the calculation of sidereal time and the solar times. However, the difference |UT1-UTC| is never bigger than 0.9 s and is ignored by Sidereal.

UTC date

The date associated to UTC.


The modified julian date, defined as MJD = JD - 2400000.5, where JD is the julian date. The julian date is the count of days since noon universal time on January 1, 4713 BCE in the julian calendar. Hence MJD count starts at midnight universal time.


The menu has the following items:

Edgar Bonet <palm@edg...>.