What's where at Lick Observatory on Mt. Hamilton

Under Construction!

Astronomical Positions of Instruments

Astronomical coordinates have been measured at 4 points on Mt. Hamilton: the Fauth Transit, the two USC&GS transits, the Meridian Circle, and the Bamberg transit. All of these instruments have been removed, and (except for the Bamberg transit) their sites have been obliterated by subsequent structures. The foundation for the Bamberg transit pier may still exist beneath the asphalt on Observatory Peak.

Measurements made with the transit instruments

Although Holden declared that the Fauth Transit defined ``the position of Lick Observatory'' there appear to be no published observations made with this instrument. It was used for the Lick Observatory timeservice in the 1880s and 1890s, but that role appears to have been assumed by the Meridian Circle by the turn of the century. Its declination scales were known to be defective in the 1890s; any measurements of longitude predate understanding of earth rotation by at least 30 years. All of the published positions for it appear to be based upon measures at other sites.

In 1888 Holden's correspondence with the USC&GS paid off when the transits were employed as part of the first Transcontinental Triangulation. These were located some 1400 feet east of Observatory Peak at the current site of the Carnegie double astrograph, but their eccentricity to the Fauth was surveyed. Though the declination transit was determined to be faulty during the observations the latitude value it produced is consistent with values at Observatory Peak to better than an arcsecond. The result from the longitude transit differs by a larger amount -- this is consistent with the accuracy of portable chronometers of that era and with the lack of knowledge of variations in earth rotation. Consequently these observations do not contribute much to this investigation.

Indications are that the Meridian Circle adopted responsibility for all transit observations before the turn of the century. Tucker engaged in 30 years of transit observations which gave a very precise astronomical latitude, but one which depends on the coordinate system of the Auwers catalog.

Lick participated in two international campaigns for the determination of longitude in 1926 and in 1933. For consistency with the other sites these were performed using a Bamberg transit setup just northeast of the Meridian Circle. The reductions of the second campaign were delayed until after seasonal variations in earth rotation were measured; these serve as the definitive measurements of the longitude of the Meridian Circle.

Usage of the astronomical position

When quoting astronomical positions of anything on Mt. Hamilton one must keep in mind the effects mentioned below. The only astronomical position for which there are useful measurements is of the Meridian Circle. All of the measurements of latitude are based on an old star catalog known to have systematic errors of as much as 0.3 arcsecond. All of the measurements of longitude were made prior to the discovery that the earth's rotation rate varies seasonally.

Tucker's published latitude of the Meridian Circle is unequivocally the best, but a systematic correction due to star catalogs must be applied to it. Because all of Tucker's original observations exist in published form it would be possible to determine a better value for the latitude using FK5 coordinates.

The longitude measurements were made before the advent of the Photoelectric Zenith Tube which has become the standard for all transit observations. BIH's published longitude of the Meridian Circle is unequivocally the best, but its validity depends on the understanding of earth rotation available in the 1930s. It is unlikely that any new reduction of the original data would produce a better result.

Transfer of these coordinates to other sites requires extrapolation across the gravity field of Mt. Hamilton. The NGS has a gravity model for the US, but it is tabulated on the geoid using a 3 arcminute grid. Even if it were not itself based on interpolations of sparser data such a grid is not sufficiently dense, and Mt. Hamilton is significantly above the geoid. The shape and height of Mt. Hamilton could easily contribute to deflections of the vertical in excess of 1 arcsecond due to curvature of the plumb line. Variations of the plumb line curvature as large as 1 arcsecond across the mountain itself cannot be excluded. In short, it is presumptive to quote sub-arcsecond astronomical positions for anything not on Observatory Peak.

The Table of Astronomical Positions

I have adopted a correction of +0.2 arcsec for the Auwers catalogs in the zones observed by Tucker. This results in an increase of latitude by half as much, or 0.1 arcsec north of the positions quoted by Tucker. I have used the BIH value of the longitude without modification.

Previous Sites of Astronomical Instruments

Meridian_Circle         +37:20:25.75       -121:38:44.115
Bamberg_Transit         +37:20:26.1        -121:38:43.440
Fauth_Transit           +37:20:25.83       -121:38:44.579

Extant Astronomical Instruments

Small_Dome (Nickel)     +37:20:26.02       -121:38:45.486
Lick_36                 +37:20:23.79       -121:38:45.99
Astrograph              +37:20:29.2        -121:38:28.1
Shane                   +37:20:30.8        -121:38:24.9

Other Landmarks

Mt_Ham_Ecc              +37:20:26.07       -121:38:46.077

Steve Allen <sla@ucolick.org>