Sky and Telescope - October 2015 - 47

Fred Schaaf

To ﬁnd out what's visible in the sky from your location, go to skypub.com/almanac.

December
solstice

O R B IT S O F THE P L ANE T S
The curved arrows show each planet's movement
during October. The outer planets don't change position enough in a month to notice at this scale.

October almost evenly spaced in a beautiful curve extending from about 30° to
15° altitude in the east an hour before
sunrise.
In the pre-dawn hours of October
8th, the crescent Moon forms a compact
triangle with Venus and Regulus approximately 2½° apart. They're still that close
the next morning, when the Moon forms
a second tight triangle with Mars and
Jupiter only about 10° below Venus and
Regulus.
On October 17th and 18th, magnitude +1.7 Mars shines less than ½° from
magnitude -1.8 Jupiter, with Venus less
than 7° to their upper right. In the same
medium-power telescopic ﬁeld of view,
Jupiter appears almost exactly 8 times
wider than Mars.
On October 25th and 26th, Venus

Dawn, Oct 17

Regulus

1 hour before sunrise

LEO
Venus

Less than
apart!

1⁄ °
2

Mars
Jupiter
Denebola

Mercury

Looking East

Mars

Venus
Mercury

March
equinox

Earth
Sept.
equinox

Sun

June solstice

Uranus

Jupiter
Saturn

Neptune

Pluto

blazes just 1.1° from Jupiter. This is the
last in the current series of Venus-Jupiter
conjunctions, one that closely resembles
the 3-2 B.C. series that may have been
the appearances that became known
as the Star of Bethlehem. Venus has
dimmed a bit from magnitude -4.7 at
October's start to -4.5; on these mornings
of conjunction, however, it still greatly
outshines Jupiter.
October 26th is also the day Venus
reaches greatest elongation from the
Sun (46° W), rising just short of 4 hours
before our star. The planet's globe may
not appear half-lit in a telescope until
several days later; check each morning.
Venus's apparent diameter shrinks from
33″ to 23″ during October - Jupiter's
grows from 31″ to 33″ in the same period
- while Venus's phase thickens from
34% to 53% sunlit.
But Venus and Jupiter have additional
close company during their conjunction.
On the morning of the 26th, the circle
containing Venus, Jupiter, and Mars is
only about 3½° in diameter. The three
travel as a "trio" - within a 5° circle -
all the way from October 22nd to the
29th. In the ﬁnal days of the month, Mars

moves away from Jupiter, with Venus in
hot pursuit. Venus soon catches Mars,
glowing less than 1° from the Red Planet
from November 2nd through 4th.
Mercury brightens rapidly early in the
month and may ﬁrst be visible about 1°
above or upper left of the slim crescent
Moon on the 11th, when the duo reaches
an altitude of about 8° a mere forty minutes before sunup. Mercury is less than
20° below Jupiter at this time, but the
gap between them grows as the month
progresses.
Mercury reaches greatest western
elongation on the 16th, when it rises at
the very start of astronomical twilight. By
the ﬁnal week of the month, the tiny
planet brightens to magnitude -1 but
descends quite low into the sunrise.

MOON PA SSAGES
The Moon pairs spectacularly with Venus
and Regulus on October 8th, and on the
9th it forms an equally splendid triangle
with Jupiter and Mars. Just before sunrise
on October 11th, the slim lunar crescent
can be spotted just below Mercury. The
waxing crescent Moon is upper left of
Saturn on the evening of the 16th. ✦
Sk yandTelescope.com October 2015

http://www.skypub.com/almanac http://www.SkyandTelescope.com

Sky and Telescope - October 2015

Table of Contents for the Digital Edition of Sky and Telescope - October 2015