Seasonal Visibility of Stars, and
Visibility of Planets in 2015-2017,
from positions of planets in their orbits
Robert C. Victor, Abrams Planetarium,
Michigan State University
These orbit charts and accompanying data
table can be used for plotting the positions of
the six inner planets, and determining any
planet’s visibility as seen from Earth. In
addition to doing the problem set below as a
desktop activity, students can “act out” each
problem’s situation in the classroom, by
having one student represent the Sun,
another the Earth, and others the five other
planets.
Be sure to have all students take a turn at
representing the Earth. That student will do
more than just stand in place, but will rotate
as well, to determine planet visibility at dusk,
in middle of night, and at dawn.
These two charts of the orbits of the planets,
one showing Mercury through Mars, and the
other Mercury through Saturn, depict the
view as seen from the north side, or “above”
the solar system. In these views, the direction
of revolution of the planets about the Sun is
counterclockwise. The outer circular scale is
labeled with values of heliocentric longitude,
measured from the Vernal Equinox, or
apparent direction of the Sun as seen from
Earth at the beginning of northern
hemisphere spring. That scale also indicates
the directions of the thirteen zodiacal
constellations (those in the plane of the
Earth’s orbit) from the Sun.
The directions of the five first magnitude
stars Aldebaran, Pollux, Regulus, Spica, and
Antares, as well as the Pleiades star cluster,
are also indicated. The outer circular scale
should be imagined to be much larger than
shown: Earth is one astronomical unit, or 81/3 light minutes from the Sun, compared to
stellar distances of many light-years. One
light-year
is
approximately
63,000
astronomical units. On a chart where the
Sun-Earth distance (one a.u.) would be
represented by one inch, a light year would
be represented by one mile.
On both orbit charts, the Earth’s orbit is
exactly in the plane of the sheet of paper.
For all the other orbits, the portion drawn as
a solid curve lies north of or above Earth’s
orbit plane. The dotted part of the orbit lies
south of or below Earth’s orbit plane.
Viewed from the north side of the solar
system, the Earth’s rotation on its axis also
appears counterclockwise. But the axis of
Earth does not point at right angles to the
plane of the orbit; rather, it tips away from
the perpendicular, leaning by about 23.4°
toward the top edge of the chart, beyond the
90° mark of the circular scale.
Using both orbit charts and the data table,
try working out the answers to these
questions:
(1) Why is the Pleiades star cluster visible all
night each year around November 20? Where
(in what direction in the sky?) would you
expect to see it at nightfall? In the middle of
the night? At dawn’s first light? Why can’t
you see the cluster for several weeks around
May 20?
(2) On what approximate date each year is
Aldebaran visible all night? Give approximate
date of all-night visibility for Pollux; Regulus;
Spica, Antares.
(3) On what approximate date each year is
Earth heading toward Antares and away from
Aldebaran? On that date, Antares is visible
(at dusk or at dawn?) about 90 degrees from
the Sun, while Aldebaran is visible (at dusk
or dawn?), also about 90 degrees from Sun.
(4) In which month would a First Quarter
Moon appear near the star Spica? Hint: The
First Quarter Moon occurs when the Moon
appears 90 degrees or a quarter-circle east
(counterclockwise in this top view) of the Sun.
(5) Which constellation is hidden on the far
side of the Sun for about the first month of
Northern Hemisphere summer?
(6) Describe the arrangement of Sun, Venus,
and Earth that occurred on August 15, 2015.
The arrangement, with Venus passing
between Earth and Sun, is called an inferior
conjunction of Venus. Notice Venus was
located in the portion of its orbit plotted as a
dotted curve, rather than solid. During the
alignment on Aug. 15, 2015, did Venus pass
north, or south, of the Sun’s disk?
Before Aug. 15, 2015, the previous time
Venus passed between Earth and Sun
occurred just over 19 months earlier, on Jan.
11, 2014. On that occasion, did Venus pass
north, or south, of the Sun’s disk? Just over
19 months before that, on June 5, 2012,
Venus appeared as a small black dot moving
across the Sun’s disk. This rare event was a
transit of Venus, which won’t happen again
until December 10, 2117. From the orbit
diagram, can you explain why transits of
Venus can happen only in early June or early
December? After Aug. 15, 2015, the next
inferior conjunctions of Venus will occur at
intervals of just over 19 months, on Mar. 25,
2017 (north or south of Sun’s disk?), Oct. 26,
2018 (north or south of Sun’s disk?), and Jun.
3, 2020 (narrowly N of Sun’s disk). For
several weeks before and after each of these
events, what will be the phase of Venus?
In 2016, Mercury passes inferior conjunction
on Jan. 14, May 9, Sept. 12, and Dec. 28.
During one of these events, Mercury will
transit the Sun’s disk. On what date?
(7) Which brilliant planets will form a close
pair on Oct. 25 and 26, 2015? (Use Outer
Planets Chart.) When will the event be seen,
at dusk or at dawn? The two planets will be
easily seen within the same telescopic field.
Describe their appearances through the
telescope. Another planet, not as bright, will
fit within the same 5° binocular field as the
bright pair, forming a trio with them for eight
mornings, Oct. 22-29, 2015. Which planet?
For much of October 2015, yet another planet
will be seen at the same time of day as the
preceding three planets, but closer to the Sun
and lower in the twilight glow. Which planet?
(8) Using the Inner Planets Chart, find which
two planets will appear close together in our
sky on Nov. 3, 2015? On Feb. 13, 2016? On
Jul. 16, 2016? For each pair, determine time
of day it will be seen, at dusk or at dawn.
(9) From late January through most of
February 2016, all five naked-eye planets will
be simultaneously seen in twilight. On Feb. 1,
2016, the Moon will appear half full and close
to one of the five planets. Plot all the planets’
positions for that date on the orbit diagrams,
and determine: (a) When can you see all five
planets, at dusk or at dawn? (b) Names of the
planets in order of their apparent positions in
the sky, from the eastern to the western
horizon? (c) Name of the planet near the “half
Moon” on Feb. 1? (d) Note in early February,
no planets will be visible at dusk, in twilight
after sunset. As the Earth rotates, which
planet rises first in the evening, after sunset?
Which rises last, shortly before sunrise?
(10) Which planet will be at opposition, visible
all night on March 7-8, 2016? In which
constellation will it appear? Which bright star
will appear about 18° west of that planet?
(11) In which constellation will the Full Moon
appear on May 21, 2016? Which bright
planet will appear near the Moon that night?
As the Earth rotates on its axis, the Moon
and the planet, near opposition that night,
will move together across the sky all night.
(12) Saturn will be at opposition, visible all
night, a day after the start of what month in
2016? In which constellation will it appear?
Which bright star will appear near Saturn?
(13) Using the Outer Planets Chart, find
which two planets will appear close together
in our sky on these dates in 2016: On Jan. 9?
On Aug. 24? On Aug. 27? On Oct. 11? On
Oct. 29? For each pair, determine the time of
day it will be seen, at dusk or at dawn.
(14) In what month in 2017 will Venus reach
its greatest angular separation from the Sun
in the evening sky? In what phase will Venus
appear then? Follow Venus’ phases through a
telescope evenings until late March that year.