Ingress And Egress

Ingress and egress are the terms usually employed for the phases when Mercury or Venus are entering and leaving, respectively, the solar disk. Such terminology may also be used for eclipses and occultations, along with their synonyms immersion (or entrance) and emersion (or emergence).

The different contact points for a transit are shown in Figure 13-2. Ingress lasts from when the planet meets the solar limb (contact I) until the instant at which the planetary disk is totally encompassed (contact II), and similarly for contacts III and IV at egress. These junctures are analogous to the contacts occurring in an annular solar eclipse, except that now the dark object is much smaller than the Moon. Without a suitable filter one cannot properly observe contacts I and IV, making accurate timings difficult, and astronomers try to time instead contacts II and III, but fixing those instants is not easy either.

This difficulty is caused by a phenomenon called the blackdrop effect. As the planet is completing its ingress, instead of a simple dark disk its image seems to be distorted into the form of a raindrop, as if a thread or ligament of material has attached it to the solar limb, pulling it out of shape. The appearance of Venus in 1769 is sketched in Figure 13-3. Contact II is strictly when that thread seems to break, with a circular silhouette being formed, completely surrounded by the Sun. Similarly at egress, for contact III, the time in question is just before the thread appears.

In visual observations the eye is often deceived. Apart from the black-drop effect, observers of the transits of Venus have reported the planet to appear surrounded by a luminous patch or aureole (Figure 13-4), sometimes with a bright spot on the dark

FIGURE 13-2. The contact points in a transit are labeled with Roman numerals from I to IV. Here M is the mid-point. The chords followed by Venus in transit across the face of the Sun during a transit depend especially upon the latitude of the observer. By measuring the times of ingress and egress accurately, the precise chord taken can be determined, and with two timings/two chords (as depicted here) from observers at known locations, it is feasible to calculate the distance to the Sun with some accuracy.

FIGURE 13-2. The contact points in a transit are labeled with Roman numerals from I to IV. Here M is the mid-point. The chords followed by Venus in transit across the face of the Sun during a transit depend especially upon the latitude of the observer. By measuring the times of ingress and egress accurately, the precise chord taken can be determined, and with two timings/two chords (as depicted here) from observers at known locations, it is feasible to calculate the distance to the Sun with some accuracy.

disk. These optical effects, due to scattering by the atmosphere of Venus, were unsuspected until transits were first watched. They limit the accuracy with which the phenomena may be timed, and that has important scientific and practical repercussions, as we will now see.

FIGURE 13-3. The black-drop effect as seen during the transit of Venus in 1769.
FIGURE 13-4. During the 1769 transit a bright ring or aureole was seen around Venus, caused by its atmosphere.
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