Early risers on Monday, December 7th in the US, Canada, and Central America will wake up to witness the Moon pass directly in front of, or occult, Venus after sunrise! With Virgo placed prominently in the southeast in the early morning hours, the occultation occurs after sunrise amid a brilliant, early morning display prominent just before sunrise. As jewels set against the dawn’s deep azure sky, we have Venus at magnitude -4.2, the waning crescent moon, Mars, Jupiter, the brilliant red giant star Arcturus, and the brightest star in Virgo, Spica.
Although all objects in the solar system (everything mentioned above except for Arcturus and Spica) move in orbital planes that are more or less aligned with the Ecliptic, the path traced out by the sun as it appears to track eastward 1º every day, an occultation of one solar system object by another is still rather rare; everything must line up. On Monday, December 7th, 2015, such an alignment occurs between the moon and Venus affording us free tickets to another spectacular show in the grandest theater of the them all, nature’s open-air theater, the sky above.
For observers along the US Eastern seaboard, the occultation begins after 9:00 AM so it occurs in daylight! Although the daytime sky is bright with the sun 45º to the east, the subjects are also bright. It is not uncommon to see the moon in the daytime sky if the air is clear and Venus if you know where to look. In this case, spotting Venus won’t present much of a challenge since it will be located along the moon’s eastern limb.
Although spectacular to view, occultations also provide unique scientific opportunities.
In the case of tomorrow morning’s occultation, in addition to measuring the angular size of Venus, a detailed, cross-sectional analysis of its atmosphere is possible at the last moment of ingress (as the moon fully covers Venus) and the moment of egress (when Venus emerges from behind the moon).
The angular diameters of some large, relatively close stars have been measured by the accurate timing of lunar occultations of those stars, a method also useful for determining their effective temperatures. In addition, discovery of binary star systems through resolution of close companions has been made during lunar occultations.
Because relatively long wavelength radio waves limit the resolution available through direct observation, early radio astronomers used lunar occultations of deep-space radio sources for determining their exact positions on the sky . This method was critical for the unambiguous identification of the quasi-stellar-radio-source (quasar) 3C-273, with its optical counterpart and relativistic jet.
Occultations of stars by planets and their moons have provided the opportunity for planetary scientists to accurately study their atmospheres, chemical composition, cross sections and dynamics. One such case was with Titan, Saturn’s largest moon, a moon whose atmosphere is thought to resemble earth’s at its infancy.
For the greatest visual impact when observing, a pair of 7×50, 9×63 or 10×80 binoculars would provide the most stunning view. Higher magnifications would necessarily limit the field of view but would allow the angular size of Venus to be measured (see above). I am going to observe this event and would welcome any observational feedback, comments or images.