Timing the Transit of Venus.
1st Contact- The instant when the planetís disk is externally tangent to the Sun (transit begins).
2nd Contact-The entire disk of the planet is first seen when the planet is internally tangent to the Sun.
3rd Contact-The planet reaches the opposite limb and is once again internally tangent to the Sun.
4th Contact-The planetís disk is externally tangent to the Sun (transit ends).
Ingress-Contacts one and two define Ingress.
Egress-Contacts three and four define Egress.