Sidereal time

[davo]

Hi, I was hoping that one or two of you could help with my question. I am try to find out when sidereal time was invented and who invented it. The reason for my question is I am doing some research on sidereal time for my local society and drawn a blank. If I can get the information together I am hoping I can make a small talk on the subject.

Thank for your help Peter.

[KeithBC]

I am sure that the ancients would have been aware of it, since several societies had accurate astronomical records, which you couldn't keep without having the concept. Your question, I think, is who first defined a zero point and formalized its measurement. I can't help you with that, I'm afraid.

[mikeyfly]

Hi i found this---->
The two natural cycles on which time measurements are based are the year and the day. The year is defined as the time required for Earth to complete one revolution around the Sun, while the day is the time required for Earth to complete one turn upon its axis. Earth needs 365 days plus about 6 hours to go around the Sun once, so a year does not consist of a round number of days; the fractional day has to be taken care of by an extra day every fourth year.

But because Earth, while turning upon its axis, also moves around the Sun, there are two kinds of days. A day may be defined as the interval between the highest point of the Sun in the sky on two successive days. This, averaged out over the year, produces the customary 24-hour day. But one might also define a day as the time interval between the moments when a certain point in the sky, say a conveniently located star, is directly overhead. This is called:

Sidereal time. A sidereal day is the time that it takes the Earth to complete one rotation on its axis so that a particular star can be observed twice at the meridian that runs directly overhead. Because the Earth is moving around the Sun as it rotates on its axis, the sidereal day is about 4 minutes shorter than the solar day, being equivalent to 23 hours, 56 minutes, and 4 seconds in mean solar time. As a result, a star will appear to rise about 4 minutes earlier every night, and different stars will be visible at different times of the year. Astronomers use a point that they call the “vernal equinox” to determine local sidereal time.

[mikeyfly]

and this--->
Historians donot know who or when mankind invented the first calendar, probably early man when he first carved a notch into a stick or a bone which marked the passing of each full moon.

Notched bones used to record moon phases have been found in Africa and Europe dating back to about 20,500 BC.


THE DAY


The civil day in ancient cultures was made up of "Watches". The length of the watch varied with the season, and were called seasonal or temporal hours. They were related to the length of the Sun’s time above the horizon. This method was known as far back as 1800 BC and was used until the end of the 13th century AD in Europe. At that time it became inconvenient to use because of the invention of the mechanical clock. The seasonal method was uneven and the mechanical clock had an even 12 hours for day and 12 hours for night. The 12 comes from Babylonian
2 x 12 = 24 (5 x 12 = 60).
They did this rather than using the base 10. This is known as the Sumerian Sexagesimal System. based on the number 60.
And it has carried to this day.
We use 60 SECONDS in a MINUTE
60 minutes in an HOUR.

The civil day now begins at midnight local time. However the Julian day still starts at noon. From the 2nd century AD until 1925 astronomers counted days from noon to noon. Primitive tribes used dawn to dawn for a day. The Babylonians, Jews and Greeks counted a day from dawn to sunset and sunset to dawn. The Hindus and Egyptians day began at dawn. The Romans and Teutons began their day at midnight.

Today the system used for the calendar is the MEAN SOLAR DAY.
Mean Solar Day is the average of a solar day because the length of the day varies slightly during the year as the Earth rotates around the Sun. The orbit of the Earth makes the Sun appear to move eastward each day relative to the Stars.
The Solar Day is about 4 minutes longer than the Sidereal Day.

A Mean Solar day is
24 hours 3 minutes 56.55 seconds
of mean sidereal time.
A Sidereal Day is
23 hours 56 minutes 4.1 seconds
of mean solar time.

Hope this all helps
mike

[tekazgtr1984]

mikeyfly: great info! Thanks for posting it!

[davo]

Thanks for the information regarding Sidereal time. It seems that it was devised a lot further back in time than I thought.

Peter

[mzu]

Thanks for posting the info Mike. Very interesting