Zenith Telescope

A night at the Gaithersburg Latitude Observatory

About one hour before beginning observations the roof of the observatory is opened to allow the temperature inside, around the zenith telescope (ZT), to stabilize.

About thirty minutes before beginning observations the telescope is brought to a vertical position and the stride level located at eye level directly above the base of the ZT is configured to be perfectly centered. The telescope is then rotated 180 degrees about the vertical axis (azimuth), the stride level is read again, and any necessary adjustments to the leveling screws of the ZT are made to make the instrument as nearly level as practical. The stride level is a very sensitive spirit level (typically 1 to 2 seconds of arc per division), enabling the ZT to be made level to within a fraction of a second of arc (one degree equals 3600 seconds of arc).

The telescope is then brought to a horizontal position pointing through a window to the south where the meridian marker is located about two hundred feet away. The center cross hair of the telescope is trained on the center of the cross hair of the meridian marker, and the position of the telescope is read. The telescope is then brought back to a vertical position rotated 180 degrees about its vertical axis (in azimuth), the stride level is checked, the telescope is again pointed to the meridian marker and its position read. These direct and reverse readings on the meridian marker will be used in processing the observations to correct for any errors in the adjustment of the optical components in the ZT. The telescope is now ready to begin observations.

Each hour a reading is taken from a sidereal clock (star time). Six thermometers and a barometer are also read and recorded. An observing list (usually recorded on a card and posted on observatory wall) describes each of the 18 star pairs being tracked for the night, including; direction of telescope, zenith circle setting, star time and star magnitude.

The ZT eyepiece assembly includes a movable crosshair, and the position of the crosshair can be read from a micrometer drum. As the star passes through the field of view the observer tracks the path of the star with the movable cross hair and readings are taken from the micrometer drum at 4 points. These readings are recorded, and the spirit levels attached to the telescope are read and recorded. The telescope is rotated about its vertical axis (in azimuth) 180 degrees, so the eyepiece is on the opposite side of the pier. The zenith circle setting is not changed.

All levels are checked and about 1 minute before the second star of the pair appears, the levels are read and recorded.

When the second star comes into view the same procedures as described above for the first star are followed. When the second star has passed through the field of view and the micrometer and level readings recorded, that concludes the observation of star pair number 1. Weather permitting, a total of 18 pairs of stars are recorded. At the conclusion of the observations the telescope is brought to a horizontal position and locked, and the roof is closed.

Each Monday photocopies are made of the observations collected during the previous week and the records are mailed to the International Headquarter in Mizusawa, Japan where they are checked, combined with observations collected at other observatories, and calculations are made to determine the position of the North Celestial Pole at various points of time. The results are distributed to astronomical observatories, geodesists, navigators, and other users around the world.

The Zenith telescope used at the Gaithersburg Observatory between 1899 and 1982 was one of four made by Julius Wanschaff in Berlin for the International Geodetic Association Latitude Service. The observatory closed in 1982. Now the Zenith Telescope is located at the Gaithersburg Community Museum.

Mr. A. W. Helm became the observer in 1957. The work of the observatory demanded intense concentration and dedication. "It gets pretty cold in here at times," Helm stated in an interview in the later 1960s. "The temperature last winter fell to seven degrees above zero." To protect himself, Helm wore electrically-heated socks, thermal underwear, and thin gloves with the fingers cut away. "It's comfortable in the summer. There's usually a nice breeze"