Mercury's orbital period around the Sun is 88 days, and its synodic period (the time from one inferior conjunction to the next) is 116 days. How often do we observe a transit of Mercury across the face of the Sun?
A)It occurs every 88 days.
B)It occurs every 116 days.
C)It occurs much less frequently than once every 116 days because Mercury's orbit is inclined 7° from the plane of the ecliptic.
D)We never see a transit of Mercury because it always crosses the face of the Sun when it is on the opposite side from Earth.

Question

Quizplus · Accepted Answer

Transits of Mercury occur when Mercury passes directly between the Earth and the Sun, so that we see it as a small black dot moving across the face of the Sun. Since Mercury's orbital plane is inclined 7° from the plane of the ecliptic (the plane of Earth's orbit around the Sun), transits can only occur when Mercury is near the points where its orbital plane intersects the plane of the ecliptic. This only happens twice a year, around May 8 and November 10, but not all of these transits are visible from Earth. In addition, the Earth must be in the right position in its orbit to see the transit, which only happens about 13-14 times per century. Therefore, transits of Mercury are much less frequent than once every 116 days.

Mercury's Orbital Period Around the Sun Is 88 Days, and Its