The Great Pyramid: Measures of Time And the Precession of the Equinoxes (cont.)
By Richard E. Ford
Reckoning in the Heavens and Counting the Stars
The British astronomer, Richard Anthony Proctor, in his book, The Great Pyramid, Observatory Tomb and Temple, recognized that the Grand Gallery of the Great Pyramid was ideally configured and precisely aligned to observe the passage of stars in the Northern Hemisphere. As an astronomer, he had no doubt that the Grand Gallery had been used for this purpose and his argument was both detailed and compelling, and as Peter Tompkins observed in his seminal work, Secrets of the Great Pyramid, Proctor's argument has never been refuted. Proctor noted that the Grand Gallery is perfectly aligned with the Pyramid's meridian, or longitude, and that the narrow slit in the ceiling of the Gallery, which runs for its entire length at an angle of 26.3° atop high corbelled walls, permits clear observation of the stars over an arc in the heavens of some 80° along the meridian as they pass overhead. This would have enabled an observer precisely to time the meridian passage of each star as it moved across this opening from east to west. He further noted that an observer in the Gallery could also have readily measured the star's declination or angular height above the celestial horizon at the same instant that it reached its upper culmination on the meridian. Afterwards, armed with the exact measure of a star's declination and its time of meridian passage converted to hour angle measurement (hours, minutes and seconds of arc), which are respectively equivalent to earthly latitude and longitude, its precise location on the celestial sphere could be affixed with ease and plotted on a flat star chart. The stars observed could also be organized into tables according to the time of their meridian passage or hour angles and declination for ready reference and to assist with further observations.
Egyptian astronomers were no doubt well aware from long practice that the time for successive appearances of a star at an observer's meridian was shorter than the time required for the Sun to make successive LAN on the same meridian. The time between successive meridian passages by a star is 23 hours, 56 minutes, 4.09 seconds, which is known as the sidereal day, while the time between successive LAN meridian passages by the Sun is 24 hours of mean solar time. The difference between the two arises from the motions of the Earth in orbiting the Sun. However, timing the meridian passage of stars is more conveniently done in sidereal time than in solar time, so the Egyptians probably adapted clocks to track sidereal time in lieu of solar time for their celestial observations of the stars. The shorter sidereal day was divided into proportionately smaller hours, minutes, and seconds, than the solar day, but still followed the 24 hours/day, 60 minutes/hour, and 60 seconds/minute unit conventions of the solar day, with the sidereal second equal to 1.00274 solar seconds. Both solar time and sidereal time are arranged in hour angles along the equator, which gives rise to what are now called time zones. This enables time to be converted to longitudinal measure and vice versa. Using hour angles, one hour of solar time and one hour of sidereal time are both equal to 15° of longitude.
The solar day begins at midnight, 00 hours and 00 minutes, each day. By longstanding convention, probably going back to remotest antiquity, the sidereal day begins when the meridian or hour angle of the Sun's vernal equinox LAN crosses an observer's meridian. In this respect, the Sun's vernal equinox LAN meridian functions as if it were a star chosen to mark the beginning of a new sidereal day. Vernal equinox LAN is the starting point, 00 hours and 00 minutes, for the sidereal day and it is prominently marked on star charts and tables. A sidereal day runs from the meridian passage of the vernal equinox LAN at an observer's meridian to its next meridian passage. The vernal equinox LAN meridian is simply its location at any given moment from an observer's meridian, as measured by the local sidereal clock (e.g. if it is 22:00 local sidereal time, the vernal equinox LAN meridian is 2 hours east of the observer's meridian). The meridian passage of each and every star can then be timed from this point forward. (It is tempting to speculate that the ancient Egyptians used the Great Pyramid's meridian as the Greenwich-equivalent of the starting point for the sidereal day and the solar day as well, but exactly which location was accorded this honor back then is not known.)
After marking the vernal equinox LAN meridian on the star charts and tables, it is then easy to determine from them which stars were present in the Sun's celestial background-the band of the zodiac-and precisely where the Sun was located with respect to this background at that instant, even though the stars were not visible in the bright light of the noon Sun. All of the stars in the band of the zodiac are visible in the night sky at some point during the year and can be observed and measured. Once comprehensive star charts and tables are prepared, repeat observations and measurements are not necessary. However, all of the meridian times will have to be adjusted to the new vernal equinox LAN meridian time. It is important to remember, though, that the Sun's sidereal year runs between successive appearances at the same vernal equinox LAN meridian, and is 365 days, 6 hours, 9 minutes, and 9.54 seconds in length. (Not to be confused with Earth's sidereal year, which is 366.24 sidereal days in length.)