##### Rotational Times

Rotational times take their name from the fact that they are measured relative to the rotation of the Earth.

Some examples are:

- Greenwich Mean Time (GMT) - This time standard is taken from the mean of the annual points at which the Sun crosses the Greenwich Meridian. It is not strictly observed any longer as there are other time standards that take account of even more variables (e.g. UT1). When the term GMT is used today it is usually as a synonym for UTC.

- Universal Time (UT) - There are a few versions of universal time, the principal one being UT1 which is a blend of observed data and computation of certain variable effects like polar motion.

##### Time Resources

- Audoin, C., Guinot, B. [2001] "The Measurement of Time", Cambridge University Press, ISBN: 978-0521003971

- Fisher, R. "Astronomical Times", available to view: http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html#UT 1

- Wikipedia Time Portal: http://en.wikipedia.org/wiki/Portal:Time