Simultaneous transits of Mercury and
Venus cannot occur now, though they can come close (in 1631, a
transit of Venus on December 7 occured just one month after a transit
of Mercury).
The orbits of Mercury, Venus and the
Earth change over time, and double events become possible tens
of thousands of years in the future as the longitudes of their
nodes converge.
Transits of Mercury and Venus occur
only a few hours apart on September 17, 13425. The
first double transit of Venus and Mercury will occur on
July 26, 69163, and the next on March 27, 224508 (J. Meeus & A.
Vitagliano, "Simultaneous Transits," Journal of the
British Astronomical Association, v114,3,2004, pp132-135. Link).
Think of these photographs not as fantasies but as precreations
of events hundreds of centuries away (the positions of the planets
on the solar disk is not exact, or even approximate; the relative
sizes of the Sun, Venus, and Mercury are exactly right).
The composite image is based on photographs
made under remarkably similar circumstances in 2004 and 2006. Here
are the individual photos:
