A Schedule of Events
its worst evening apparition of the year for observers in the
Northern Hemisphere. It reaches greatest eastern elongation from the
Sun - 26° - on September 21st. However, the ecliptic lies
at such a shallow angle to the horizon that Mercury is only 7° above
the horizon at sunset, and 3° high a half hour later when the sky
might be dark enough to spot it. It retains this altitude for
several days before and after this date. Mercury is actually quite
bright at magnitude 0.0, so it might be easy to spot in binoculars
in spite of its low altitude.
getting increasingly low in the western sky after sunset. On Sept. 1st,
the planet lies 24° above the western horizon a half hour after
sunset; by the 30th, its altitude is just 13° 30 minutes
after sunset. At midmonth its disk spans 16 arc-seconds, while its
rings stretch 37”, and are tilted 22° to our line of sight. This
month represents the last opportunities to get good views of Saturn,
so take advantage of the opportunity! When will Saturn return?
Please check our “What’s New” page to find out!
On September 1st, Mars
lies just 5° to the right of Saturn, but the gap widens as Mars
moves eastward. On the 27th, Mars passes just 3° to the
south of its similarly colored rival, Antares. (The star’s name can
be translated as “rival of Ares” – the Greek name for Mars). The
view of Mars through a telescope is disappointing, as it is only
about 6” across – far too small to reveal any surface detail.
reached opposition on August 29th, which means it was at
its closest, brightest, and largest of this entire year. Throughout
September, it remains a prime viewing target. At magnitude 7.8, the
planet is well below naked-eye visibility, though it can easily be
picked up in binoculars if you know where to look. In a telescope,
it shows a tiny blue-grey disk about 2.4” across.
Our Solar System’s other “ice giant”,
Uranus, follows Neptune across the night sky. It rises
about an hour and a half after Neptune, in the constellation Pisces.
It will not reach opposition until early October, but it is already
near its peak magnitude of 5.7 (bright enough, incidentally, to be
seen with the naked eye under good conditions). In a telescope, it
displays a clear greyish-green disk about 3.7” wide.
two and a half hours before the Sun as September begins, and four
and a half hours before sunrise by month’s end. It shines at
magnitude –1.8, and is increasingly easy to see as it climbs higher
day by day. In a telescope, it displays a disk 33” across, large
enough to show detail even when the planet is low. Its four large
moons – Io, Europa, Ganymede, and Callisto – are visible even in
small telescopes, and you can follow their changing patterns night
As September begins, Venus
rises about an hour and a quarter before the Sun, and lies about 15°
to Jupiter’s lower left. Its brightness – magnitude -3.9 – makes it
hard to miss even in the morning twilight. As the month goes on,
however, it rises later and later; by the 30th, it is
virtually lost in the solar glare.
At midmonth, the dwarf planet/asteroid
1 Ceres is about 5.5° to the upper right of Saturn at
Pluto lies due north of the
“Teapot” in Sagittarius; It shines no brighter than magnitude 14.1,
and represents a difficult target.
When Will Saturn Return?
I have the privilege of working at two
major observatories in the Boston area, and I get a variety of
questions from the general public as they are looking - or, more
commonly, waiting to look! – through a telescope. One of the
most common questions is whether there are more planets visible
in the summer or winter, or whether they planets we are looking
at are visible at the same time every year. The answer is: well,
In order to really explain the cycles
of the planets, we need to introduce the idea of “syndonic
periods”. A syndonic period is the time it takes a body to
return to the same location relative to the Earth and Sun, i.e.,
the same point in the night sky.
Let’s consider the case of Mars.
Obviously, Earth and Mars both orbit the Sun, in different
periods of time. When the Sun, Earth, and Mars are positioned in
a line, Mars is considered to be in opposition. When does the
next opposition occur? Earth, being the inner of the two
planets, moves faster and completes an orbit more quickly –
every 12 months. But by the time Earth has returned to the
position of the previous opposition, Mars will have moved on
some additional distance along its orbit. Earth will have to
move for twelve months plus the additional distance Mars has
travelled to duplicate the conditions of the opposition.
There is a formula to calculate such
“syndonic periods” among the planets.
Earth’s year is, of course, 1 year
long. That of Mars is 1.88 years. The formula for calculating
the syndonic period is as follows:
= 1/P1 – 1/P2
where P1 is
Earth’s period and P2 is that of Mars.
Plugging in the values above, we find
the syndonic period of Mars to be about 2.1 years. (This is
actually an approximation, since the orbits of Earth and Mars
are non-circular and not co-planar.)
An interesting effect is that, the
farther the planet is from the Sun, the closer its syndonic
period comes to being 1 year – since the planet moves more
slowly as its distance from the Sun increases.
Here is a table of synodic periods in
the Solar System, relative to Earth:
Sidereal Period (“Year”)
From the table we can see that Saturn
returns to its same approximate position in our sky after about
1.035 years, or about 12.42 months.
The positions of the planets in our
sky, as seen from our observatories, returns to the same place
after approximately one syndonic period. So there is the answer
to the visitors’ question – albeit a somewhat complicated one!
September 18, 2014 Star Chart
9:00 PM EDT
Looking at Zenith, South at Bottom
After dark, the sky is bereft of planets visible to the
naked eye. Uranus and Neptune are prominently placed, but they
require optical aid and star charts to detect – or, better yet, a
“go-to” telescope. The “Summer Triangle” – comprised of Vega,
Altair, and Deneb – is almost directly overhead. .The Milky Way
still streams across the sky from SW to NE across the sky. The chart
above shows more stars than are visible from most urban or bright
suburban skies; under dark sky sites – increasingly rare in most
areas of the Eastern U.S. - many more stars would be visible.