Download July 2015 - Faculty

The Night Sky
A Blue Moon Month
As mentioned last month, Venus and Jupiter will be very close to each other on the sky over the last part
of June and first part of July. At our location, Venus and Jupiter will be closest together, only 0.3
degrees apart, on the evening of June 30th. The next evening, July 1st, they will still be very close
together, though now at a separation of 0.6 degrees. With each new evening in July, these planets will
continue to separate. The earth is pulling away from Jupiter as they both orbit the sun, though Venus is
catching up to us in its orbit. Due to this, both of these planets will continue to sink in the western sky
as the month progresses. Both set 2 hours and 20 minutes after sunset at the beginning of the month,
but by month’s end, Venus sets 30 minutes after sunset and Jupiter 1 hour after sunset.
This close planetary conjunction occurs in Leo near the bright star Regulus. It has been speculated that a
similar conjunction between these two planets in the constellation of Leo in 2 B.C. may have been the
Star of Bethlehem mentioned in the Bible.
On July 14th, Venus makes its closest approach to Regulus with a 3 degrees separation. A few days later
on July 18th, Venus and waxing crescent moon will be close together on the sky, with Regulus a few
degrees above, and Jupiter visible to the right of this grouping. Also on this date, the New Horizons
spacecraft makes its closest approach to the dwarf planet Pluto. This will be the first time humanity will
get a close up view of this famous planet. Venus will be at maximum brilliance from July 11th through
the 18th. During last half of July, Venus’s crescent should be easily visible in low power binoculars.
Saturn will be visible all evening in the southern sky during July. It is in the constellation of Scorpius just
to the upper left of the bright red supergiant star Antares. On July 25th, a waxing gibbous moon will be
just a few degrees from Saturn. Coincidentally on this date, the dwarf planet Ceres will be at opposition.
Ceres was the first of the asteroids to be discovered. It is too faint to be seen with the naked eye,
though one can easily spot it with binoculars if you know where to look. It should be pointed out that
the Dawn spacecraft is currently in orbit around this dwarf planet. As such, this month of July 2015 has
two close encounters between a NASA spacecraft and a dwarf planet.
Mercury and Mars have a close conjunction (0.1 degree apart) on July 16th, however this conjunction will
be difficult to see in the bright morning twilight. Note that both of these planets will rise only 45
minutes before sunrise on this date.
The month of July 2015 will have two full moons visible, the first on July 1st at 10:20 p.m. EDT and the
second on July 31st at 6:43 a.m. EDT. When a month has two full moon, astronomers often refer to the
second full moon as a “Blue Moon,” even though the moon does not actually looks bluish in color. The
earth is at aphelion, the farthest point in its orbit about the sun, on July 6th at 3:41 p.m. EDT. On this
date, the earth will be 94,506,507 miles from the sun, as measured from center to center.
On July 30th, the Delta Aquariids meteor shower will peak. This is a long lasting meteor shower, though
a bright moon may interfere with seeing any of these meteors this year.
The ETSU Powell Observatory open houses are on hiatus until September. Later this summer, the 20152016 schedule for our Astronomy open houses can be found on the web at
http://www.etsu.edu/cas/physics/observatory/default.aspx.
This month’s Night Sky was written by Dr. Donald G. Luttermoser, Chair of the Department of Physics
and Astronomy at ETSU. He can be reached at [email protected]. Astronomy-related information
for the public, including a link to the ETSU Powell Observatory, can be found at
http://www.etsu.edu/cas/physics/outreach/astronomy.aspx.