# Recording the observations in Universal Time

When Venus crosses the Sun, we need to note the times of the contacts between Venus and the Sun, at the beginning and at the end of the transit. These observations will be gathered to estimate the distance between the Earth and Venus. In order to combine the observations, it is essential to measure the events using the same time scale. The scale adopted for all observers will be Universal Time (abbreviated UT).

Civil time as shown on our watches is ultimately derived from the rotation of the Earth and the daily passage of the Sun across the sky. Local noon or midday occurs when the Sun is at its highest position in the sky. But the Sun does not move evenly across the sky because the Earth's orbit is an ellipse and the Earth's axis is tilted. Therefore we use the concept of a fictitious body, the mean sun, which crosses the sky at a uniform rate above the Earth's equator. We can establish a universal time scale by relating events to the passage of the mean sun relative to the Prime Meridian of Greenwich at zero longitude.

We make thus the assumptions that each day begins at midnight (00.00 hours) and lasts 24 hours or 86400 seconds, and that the Earth's surface is divided into 24 time zones in each of which civil time differs from Greenwich Time by an integral number of hours. When it is noon (12.00 hours) at Greenwich, it is 12.00 hours plus (or minus) N hours in the countries situated at N time zones east (or west) of Greenwich.

To obtain Universal Time directly, we just need our country's civil time, a time which differs by a whole number of hours from Universal Time. Civil time can be easily accessed by phone thanks to the speaking clock. Some radio stations like Frankfurt and Rugby broadcast Universal Time; commercial clocks can be synchronised to these signals. GPS receivers can also provide Universal Time.

Equipped with a clock adjusted to Universal Time, or differing from it by an integral number of hours, we are ready to time each event when Venus crosses the Sun. The time should be estimated while the event takes place or, if you record it on video, interpolated from the individual frames. You should attempt to record it to the nearest second . This should be the precision of the observation.

A precision of one second is more than adequate but one minute is not sufficient. So we will record the event to the nearest second of time. If several people observe at the same time, each should not be influenced by the observations of the others. It is normal that neighbouring observers do not record a phenomenon at the same moment since observing methods may differ. Statistical methods will be able to decide between observers. Averaging over a very great number of observers will make it possible to obtain a result more precise than each individual measurement.

If you record your observation with a video camera or a webcam, for example, timing will be easier. First of all, try to record on each image the hour, the minute and the second of Universal Time or civil time (or record a speaking clock on the sound track). If this is not possible, note the difference between the recorded time and Universal Time with a precision better than a second. If each image is timed accurately, it will be easy to interpolate the times of the images to obtain the time of contact.

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