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Leah Rachel Nellie Solar Telescope Transit Telescope Makers Workshop

Meridian Transit Telescope

Above:  The Chabot Transit Telescope in its housing at Mountain Blvd., Oakland, California. Photo credit: Jim Ferreira

The Earle Linsley Meridian Room at Chabot Space & Science Center, named in honor of Chabot’s second director, houses a 4 1/8-inch Fauth Transit telescope.

In 1885, the transit telescope, along with solar and sidereal clocks, was installed at the original Oakland Observatory, later known as Chabot Observatory. Astronomers made timekeeping observations every Friday night, setting the official time for Oakland. The instruments automatically rang a bell at Oakland City Hall twice a day. The telescope was moved in 1915 to Chabot’s Mountain Boulevard site, where it remained in operation until its relocation in 2000.

Why Use a Telescope to Tell Time?

A hundred years ago, ships used the time and measurements of star positions in the sky to determine their location. But mechanical clocks of the day were not very accurate, and time was of the essence in calculating geographic longitude.

We can observe the Sun and other stars traveling across the sky at a constant rate each day.  By knowing one’s exact position on earth, as well as a star’s position in the sky, astronomers would mark the time when a star passed or “transited" an imaginary line in the sky north to south, the “meridian”. Sea captains regularly reset their navigational clocks using standard time based on these meridian transit observations. Knowing the exact time remains very important to navigation, even today.

The Big Clock in the Sky

The first clock was the sky, the Sun the hour-hand.  What is a clock but a numbered dial with hands that move constantly around it, their positions indicating the time? Though as a “clock” the sky is very accurate, people’s ability to read the exact time by simply looking at the position of the Sun is limited.  You can tell when it is around noon by noting when the Sun reaches its highest point in the sky, when it crosses the meridian.  But how easily can we mark the precise moment when this event occurs?

The answer to this is to use a tool—and it doesn’t take high technology to find the precise moment when the Sun crosses the meridian.  A stick casting a shadow on the ground declares noon when the shadow is shortest.  A hollow tube pointing exactly at the correct point on the meridian announces noon when we see the Sun through it.