Time - NASA
eclipse.gsfc.nasa.gov › LEcat5 › timeApparent solar time was eventually replaced by mean solar time because it provides for a uniform time scale. The key to mean solar time is the mean solar day, which has a constant length of 24 hours throughout the year. Mean solar time on the 0° longitude meridian in Greenwich, England is known as Greenwich Mean Time (GMT).
Solar time - Wikipedia
en.wikipedia.org › wiki › Solar_timeApparent solar time can be crudely measured by a sundial. The equivalent on other planets is termed local true solar time (LTST). The length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes.