Observing

Observing the transit

William Morris observing at Jimbour

By carefully following the timing technique outlined below, you may contribute to the reenactment of an appealing episode in astronomical history! It should be stressed that the guidelines are intended for visual observers only, not for deducing timings from a videotape or digital-image sequence. The duly timed contacts may throw light on the inherent influence of the atmospheric blurring as well as the observer’s personal equation upon the measurements’ uncertainties, and therefore help us to clarify the interpretation of timings made in past centuries.

What instruments to use

The simplest and safest way to observe the transit of Venus is by projecting the sun’s image, for example using the Solarscope. Venus will be seen as a small black dot on the solar disk. The advantage of a projected solar image is that many people can watch the phenomenon at the same time – it’s perfect for providing large crowds a peek at the transit in a convenient way. A certain disadvantage however is that accurate timings of the interior contacts are almost impossible.
In order to establish a reliable solar parallax from visual observations of the times of contact, these contacts should be timed with a precision of at least one second. From experiences with the 2004 transit of Venus, you are advised to use a telescope on an equatorial mounting with a drive. The telescope must be equiped with a proper solar filter that fits the full entrance aperture of at least 120 mm. Make sure you use a safe solar filter. Furthermore, apply a power of at least 40 times. A large aperture and magnification will much reduce the telescope’s influence on the black drop effect, leaving the atmospheric turbulence as the main cause of uncertainty.
Furthermore, you will need a good stopwatch to make accurate measurements. You can use your local computer clock, which must be synchronized via the internet. You can also use the STOPER software, developed by Polish amateur astronomer Arkadiusz Dudka. Also, you need to know your geographic coordinates with a precision of at least one arc minute, for instance by using Google Earth or GPS on an iPhone or digital camera.

Timing the transit

At each contact, start looking at the foretold position of sun’s limb about two minutes before the predicted time of contact, in order not to fatigue your eye. You can make your own contact predictions for any location on this website.

Just before interior contact at ingress the sun will show two fine sharp horns of light, gradually approaching each other to meet around the dark body of the planet silhouetted inside the sun’s limb. The moment of contact is regarded as the time when the sharp points of the cusps begin to meet. But the ends of these cusps assume a certain haziness and their ill-defined outline will often be wavy and diffused by the effect of atmospheric tremors. The moment of interior contact at ingress is that at which light is about to glimmer all the way across the dark space between the cusps. Very generally there will be a period of several seconds during which the light will glimmer and disappear by turns. The middle of this period is that of true contact.
Timing this contact requires that you record two times. Rather than waiting for the cusps to unite, record the time every few seconds (no contact yet) untill sunlight begins to sweep intermittently all the way across the dark space (contact occurred). Midway between your last two recorded times is the time of interior contact at ingress.
At egress, all the phenomena described above will recur in reverse order. As the planet is approaching interior contact at egress, the band of light in front of it, as it fades away, very gradually grows fainter until it is almost lost in vision. The time of interior contact is marked by the moment at which light entirely ceases to glimmer across the dark space formed by the approach of the planet’s limb to that of the sun. From the commencement of the intermittently darkening until the definite disappearance of the sun’s light from view, there is a series of progressive changes, which may extend over a period of several seconds. Again, the middle of this period is that of true contact.
Timing this contact also requires that you record two times. Rather than waiting for the light to disappear, record the time every few seconds (no contact yet) untill sunlight begins to disappear entirely all the way across the faint light space (contact occurred). Midway between your last two recorded times is the time of interior contact at egress.
However, the bright sunlight refracted by the atmosphere of Venus, that appears on the planet’s limb silhouetted against the dark sky during ingress and egress, should not be misapprehended for the true light of the sun’s limb. The distinction between the two is a matter of judgement which must be left to you. Also, you must be especially careful to distinguish what is permanent from what is an accidental product of atmospheric vibration.