TIME
What time is it? The first definition of time was probably the (local) hour angle (LHA) of the sun, indicated by a device known as a sundial. Actually, rather than the LHA of the sun, we use the LHA of the sun plus twelve so that when the sun crosses the meridian (meri=half, diem=day) and has LHA=0 (noon) the "time" is 12. This is for convenience so the date changes at midnight when most people are asleep. (Of course astronomers work at night so you will often see event times listed as e.g. "11 PM to 2 am Aug 11/12".) Sundial time is called local apparent solar time LAT=HA(sun)+12.
Sundials east of your longitude will read four minutes earlier than your sundial for each degree of longitude east. So Brisbane (153°00'E) sundials will read noon twelve minutes before a sundial at longitude 150°E. Time is longitude. That is why the word apparent is included in the definition, the time depends on your longitude and actually relates to the real sun. Fortunately, this is not how time is now defined at all: Time is defined to make motion look simple and the motion of the sun across the sky is very complex.
Atomic time is defined by the atomic second which corresponds to 9,192,631,770 periods of the ground state hyperfine transition in cesium-133. And note that the metre is now defined to be 1/299792458 of the distance light travels in one atomic second in vacuum. The speed of light c=299792458 m/s is now a defined quantity). Our timepieces are set to run at the atomic rate and since this differs from the solar rate (the earth is gradually slowing down so there are more and more atomic seconds in a day as time goes on) we occasionally insert a "leap second" (midnight, June 30 and/or December 31)to keep our clocks synchronized with the irregular solar time.
The motion of the sun across the sky arises mostly from the eastward prograde rotation of the earth (which causes the sun to rise in the east and set in the west) with a rotation period of one sidereal day, 23h 56m 4.091s. Too bad we use solar and not sidereal clocks, since this star rate is quite uniform though the earth is gradually slowing down due to tidal friction. The star day is about four minutes shorter than the solar day so the stars appear to rise four minutes earlier each day (a half hour earlier a week, two hours a month, 24 hours a year).
The prograde revolution of the Earth about the sun causes the sun to rise in the west and set in the east with a period of one year. If the earth did not rotate the sun would rise in the west and the day would be one year long. If the earth rotated prograde once per year, the sun would never rise or set, a not uncommon situation known as synchronous rotation. Like the Moon.
The revolution introduces two drastic effects that affect the rate of the "true" sun across the sky. First, the axial tilt of the earth introduces a north-south component to the solar motion that is most rapid near the equinoxes (the seasons switch fast) and slowest near the solstices (summer and winter are long). So the projection along the equator (hour angle motion) is slow around the time of the equinoxes and fast around the summer and winter solstices, compared to a hypothetical ideal mean sun that moves along the celestial equator at a constant (consistent with the definition of time) rate. Second, the eccentricity of the Earth's orbit causes the revolution rate to vary, fastest near perihelion, slowest near aphelion.
The combined effects cause the true sun to lead or lag the mean sun by the equation of time, ET, the difference between the local apparent and local mean solar time: ET=LAT-LMT and is up to plus or minus 15 minutes.
Since it is inconvenient for time to be a function of longitude, the world is divided into time zones. Usually the zones are set to longitudes that are multiples of 15°. For example, in Brisbane we set our watches to the LMT of longitude 150°E (just about where Banana Qld. is located). Zone time is called standard time. So Brisbane LMT is 12 minutes earlier than your watch reads. So noon (for the mean sun) always occurs at 11:48AM.
Local time = Zone time + longitude correction of 4 minutes per degree.
Subtract 4m per degree if you are east, add 4m per degree if you are west of the standard longitude (150° for Queensland).