What is the key difference between LST and ZT?
Also, are UT, GMT and Greenwich Mean Solar Time the same?

  • $\begingroup$ I recently posted a link to a bunch of articles about various time scales in this comment on the SE Physics site. Happy reading! :) $\endgroup$
    – PM 2Ring
    Apr 10, 2019 at 11:58

2 Answers 2


Universal Time (UT) and Greenwich Mean Time (GMT) are based on the average (mean) position of the sun. The mean position of the Sun crosses the meridian every 24 hours. There are subtle differences between UT and GMT, but they are the same for most practical purposes. (The Wikipedia article for Greenwich Mean Time points out that the start of the day under GMT changed from noon to midnight sometime in the past. The Time and Date.com article GMT versus UTC describes GMT as a time zone and UTC as a time standard.)

Since our clocks are also based on the mean position of the Sun, the time in your time zone is a fixed number of hours before or after UT.

Local sidereal time (LST) is the time according to the stars at your longitude. Let's assume that star X is on the meridian at 0 hours (midnight). The same star will return to the meridian after 23 hours 56 minutes 4 seconds (approximately) of clock time. In other words, the star time runs almost 4 minutes fast each day. Therefore, there are two key differences between sidereal time and the clock time:

  1. Sidereal time is different at each longitude. Clock time is the same for all longitudes within a time zone.
  2. To convert from clock time to sidereal time, you need to know the time and date.

Solar time is the ordinary 24 hour day. Local 'noon' is the time when the Sun is located at the central meridian on the sky (the mid-point separating the eastern and western sides of the sky). With timezones, we cheat a bit by grouping everyone in the same timezone to agree to use the same time on the clock ... whether or not the Sun is actually at the central meridian at noon (for most observers it will be near the meridian ... but not at the meridian). There are other nuances since the Earth does not travel at a constant speed in its orbit around the Sun. The elliptical shape of our orbit causes the planet to slightly speed up or slow down at parts of the year. We don't adjust clocks for this ... we just ignore it. That means even if you live at a position in the timezone where the sun actually is at the meridian at noon on one day ... several months later it is not likely to be at the meridian at noon. If you were to photograph the Sun at noon each day of the year for an entire year (assuming you never have cloudy skies), and combined the images, the shape you would see in the final merged image would show a path in a figure 8 on the sky. The altitude of the Sun above the horizon changes based on season due to Earth's tilt. But the east/west variation is based on the changes in Earth's velocity through space as we orbit the Sun. This shape is called the Sun Analemma.

Sidereal time is based on the amount of time it takes Earth to complete one revolution on its axis. In other words if you select a star other than the Sun and determine how long it takes for that star to move from the mid-point in the sky one day until mid-point in the sky the following day ... that length of time is a sidereal day. In liberally rounded values... it's roughly 4 minutes short of a 24 hour solar day. (approximately 23 hours 56 minutes 4 seconds)

The difference occurs because in the time that it takes the Earth to complete one rotation on its axis, the planet also moves forward in orbit around the Sun. There are roughly 365 days per year and 360 degrees in a circle. This means the Earth will have progressed just shy of 1 degree in it's orbit (our orbit is not a perfect circle so there are times of the year when the planet is moving slightly faster than other times of the year ... but on average it's just a tiny bit short of a degree.) Even though we've completed an entire revolution on our axis, the Sun wont return to the same position because the Earth has progressed in its orbit. We need to let the Earth keep spinning for nearly 4 more minutes to get the Sun to return to the same position in the sky.

This fact causes the local sidereal time to diverge from solar time by roughly 4 minutes per day. In 15 days your solar vs. sidereal clocks will have diverged by about an hour.

The 00 hours point is also called the "First point of Aries" (https://en.wikipedia.org/wiki/First_Point_of_Aries). To make things more confusing ... the point is actually located in Pisces ... thanks to the precession cycle of the Earth's axial wobble (when the position was first established ... it actually was in Aries)

The First Point of Aries is defined as the position of the Sun in the sky when the Sun passes from the Southern Hemisphere to the Northern Hemisphere at the Vernal Equinox (the start of Northern-Hemisphere Spring / Southern Hemisphere Fall) in March. That point becomes the "00" hour point. The right-ascension coordinate of all objects in the sky is based off that point.


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