A day is 24 hours -- this we all take as an incontrovertible fact, uncontested as much as how we know the sun rises in each morning and that if we drop a cup, it will fall to the ground.
The earth's day is measured as the time it takes the planet to make one complete rotation. But a day is not 24 hours exactly. In fact, the duration of a day only fluctuates around the time period of 24 hours.
The length of a day varies for a number of reasons, and there are different time scales to account for all of them. So, while we have something called Universal Time to mark the official time of day, there are, in fact a number of different forms of Universal Time.
Universal Time 0 (UT0) takes into account some basic irregularities of the earth's rotation around the sun.
The earth does not travel around the sun in a circle. Rather, its path is an ellipse, one that draws closer to the sun during the winter (in the northern hemisphere). As the earth draws closer to the sun, it spins slightly faster. Also, the tilt of the earth's daily orbit is not even in relation to its orbit around the sun. It's slanted about 23 1/2 degrees.
The earth also wobbles, so to speak. Each year the locations of the south and north poles shift, which cause the length of the day to fluctuate by as much as 30 milliseconds.
The second form of Universal Time, UT1, adjusts UT 0 for additional fluctuations. For instance, the earth's rotation seems to be slowing down. The day about 16,000 milliseconds longer than it was 1,000 years ago. The day was only about 21 hours around 600 million years ago, studies of fossil corals show.
UT 2, The third form of Universal Time, adjusts UT 1 for pull of tides, another factor. Thanks to the pull of the tides, water splashes about the planet. In the winter, water freezes up in the mountains and around the poles, slowing rotation ever so slightly. When spring comes it melts down into the seas, quickening earth's rotation. Because Most of the mass on the planet is in the northern hemisphere--the earth is top heavy if you will--the seasonal activities there impact the planet the most.
Coordinated Universal Time (UTC), today's official time, is based off of UT2.
Now, what if we measured a daily orbit not against the sun -- using two succesive high noons to measure a day-- but rather against a star further away. In this case, the length of the day would be measured against when a star would appear in the same location each night. This time measurement is called Sidereal Time.
Sideral Time varies slightly each day as well, due to where the planet is in its orbit around the sun on a given night, in relation to the star it is being measured against. But a Sidereal day is always shorter than mean solar day by about 4 minutes.
While the UT variants go to great lengths to account for all the planet's fluctuations, another tact has been taken that just ignores the slight unsteadiness of earth's rotation altogether. This approach is called Ephemeris Time, ET.
Ephemeris Time provides the time as it should be, one unruffled by the earth's wobbles. The challenge with ET is that, since its based on time among different astrological events, it can take some time to determine the exact time.