University of Michigan - Department of Astronomy




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Timekeeping and Telescopes at the Detroit Observatory

The soul without imagination is what an observatory would be without a telescope.

--Henry Ward Beecher



Solar Time

Since before recorded history began, people have measured time. It seems like a very simple thing; we have clocks all around us that tell us what time it is. However, setting our clocks is not especially simple. How do we define the units and reference frame for measuring time? This originates from astronomy.

Our clock is based on the day. The day is the average time it takes for the Sun to return to the local meridian, which is the line of right ascension (pole to pole) passing through the zenith (overhead). For historical reasons, the day is divided into 24 hours. Solar time is the time you measure from a sundial (Figure 1). Sundials track the apparent motion of the Sun during the day, usually by using a shadow, although an aperture against a shadowed background also works. A shadow is shortest at midday, when the Sun is on the meridian. Since the Sun rises and sets at different relative times throughout the year, midday is therefore the most accurate moment to distinguish between days. However, it would be inconvenient to change the date at midday, and so we start our day at midnight, exactly 12 hours before noon.

A sundialFigure 1: The sundial at Angell Hall. The shadow of the gnomon indicates it is just after 10 AM local solar time (click the image for a bigger version).


One big problem is that the meridian is a local reference. A person west of you has a meridian west of you, so when it is your noon, solar time, it is still morning for that other person, even if s/he is standing only a few feet away from you. Similarly, it's already afternoon for someone standing a few feet east of you. This problem is resolved by defining the time zones, which standardize everyone to the same clock within each time zone. In the 19th century, the advent of railroads made accurate, coordinated timekeeping essential, so that multiple trains could run on single tracks without disaster. The first official time zone was adopted in Britain by the railways in the 1847, based on solar time on the meridian of the Royal Greenwich Observatory. The average solar time at Greenwich is Greenwich Mean Time (GMT). The globe is divided into 24 integer-hour time zones, and there are others offset by fractional hours. In 1883-4, much of the world adopted the Greenwich Meridian as the prime meridian for both longitude and time zones, and the American railways also adopted this Standard Railway Time system. For geopolitical convenience, the time zone boundaries do not exactly correspond to an equal division of the globe by longitude. This standardized time, which is the same as the time on your watch, is known as civil time. The hours are set so that the Sun crosses the meridian at noon, in roughly the middle of the time zone, on standard time.

Today, the US Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST) maintain and provide the global standard time. Solar time is difficult to track accurately, because the Earth does not rotate perfectly uniformly. In 1967, NIST redefined the length of one second to be based on the vibrational period of a cesium atom, rather than on the subdivision of solar time. This produced Coordinated Universal Time (UTC), which is based on GMT, but at the rate of the cesium atom. The new time scale does eventually lose synchronization with solar time, and when the difference is 1 second, a leap second is introduced.

Sidereal Time

The Earth rotates relative to the distant stars, as well as relative to the Sun. Thus, we can also use the stars to tell time, which is called sidereal time ("sidus" is a Latin word for "star"). The sidereal day is the length of time it takes the Earth to make one complete rotation relative to the fixed celestial sphere. Since we defined solar time such that our local position faces the Sun every day at noon, sidereal time must necessarily be different because of the Earth's orbit around the Sun. The Earth orbits the Sun in the same direction that it rotates, so it takes slightly more than one Earth rotation for the Sun to return to the meridian between days (see Figure 2). This amounts to one extra rotation relative to the fixed stars, or one extra sidereal day, per year. There are roughly 365.25 solar days per year, and there are1440 minutes in a solar day (but see below). From this, you can show that the sidereal day is about 4 minutes shorter than the solar day.

Figure 2: Top-down view of Earth and Sun from one solar day to the next (not to scale). Noon on the solar day is defined by the observer's meridian facing the Sun (dashed lines). On the first day, this condition means that another observer at position M is exactly facing star X at midnight. On the second day, we see that for the first observer at solar noon, the second observer at M is no longer facing star X. Thus, to execute one full solar day requires one full sidereal day plus a bit more rotation.


Just as with solar time, the most convenient local reference for sidereal time is the meridian (see Figure 3). The apparent motion of the stars occurs strictly east-west because it is caused by the Earth's rotation. Thus, sidereal time is measured only by the transiting hours of right ascension (RA) on the celestial sphere. For simplicity, the RA on the meridian is equal to the sidereal time. Therefore, you can also determine the RA of an object by noting the sidereal time when it crosses the meridian. The meridian of RA = 0 is set at the vernal equinox, the Sun's position relative to the fixed stars, on the first day of spring in the northern hemisphere. When the Sun crosses your local meridian at noon on this day, the sidereal time is exactly 0:00 hours. (The sidereal day is not offset to midnight.)

Figure 3: The area of the sky visible from Ann Arbor and similar latitudes.

Coordinate System grid

The Detroit Observatory

Because the Earth's orbit around the Sun is an ellipse, and an imperfect one at that, the days vary in length and it is difficult to accurately calibrate solar time. In contrast, sidereal time is more straightforward to track and calibrate. Thus, most official timekeeping was done by astronomical observatories, based on sidereal time, and converted to solar time.

UM President Henry Philip Tappan directed the construction of the Detroit Observatory in 1854 to establish the fledgling University as a modern research institution. Timekeeping was a subject of active discussion then, important for both science and commerce. Only a few years earlier, the USNO in Washington, DC, had built their official time ball, which dropped at noon every day to signal the time to sailors and citizens. President Tappan persuaded wealthy Detroit merchants to sponsor construction of the campus observatory to provide timekeeping for the Michigan Central Railroad. This would guarantee accurate railroad operation upon which the merchants' businesses depended, and the facility was named the Detroit Observatory for them.

Timeball at the US Naval ObservatoryFigure 3: The timeball at the US Naval Observatory in Washington DC.

The Meridian Circle Telescope is designed to accurately measure the sidereal time. The astronomers confirmed the sidereal time every night (weather permitting), and they could then convert it to solar and civil time. A signal was sent by telegraph to the Michigan Central Railroad terminal in Detroit, and from there, it was forwarded to all the other connecting terminals.

In addition to the Meridian Circle Telescope, the Detroit Observatory is home to a general purpose instrument, the 12.6-inch Henry Fitz Telescope. Henry Fitz, Jr. was the earliest American to manufacture telescopes commercially, and the UM telescope is the only remaining Fitz telescope that is is operational with its original optics. When it was constructed in 1854, this telescope was among the best in the world. Detroit Observatory was the first, flagship research facility at UM, and the Fitz Telescope served the students and faculty as a research instrument.

The Gregorian Calendar

People are interested not only in the passage of days, but also in the passage of the seasons, and therefore, years. The year, which is the Earth's orbital period around the Sun, is unrelated to the Earth's rotational period. However, we need civil time to synchronize with the yearly calendar. There are 365.2422 days in an average year, so almost 365 and a quarter days. In 1582, Pope Gregory XIII established the Gregorian Calendar, which is in use today. In this system, every 4th year adds one leap day, except for years that are divisible by 100 and not divisible by 400. For example, the year 2000 was a leap year, but the year 1900 was not. This exception helps correct for the difference between 365.2422 and 365.25 days per year.

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Updated: 08/28/14 by MSO, SAM

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