Download Seasonal Visibility of Stars, and Visibility of Planets in 2014

Seasonal Visibility of Stars, and
Visibility of Planets in 2014-2016,
from positions of planets in their orbits
-- Robert C. Victor
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 each of 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.
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 of the chart or the 90° mark on 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 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 dawn?) about 90 degrees from the
Sun, while Aldebaran is visible at (dusk or
dawn?), also about 90 degrees from the Sun.
(4) In which month would a Last Quarter
Moon appear near the star Spica? Hint: The
Last Quarter Moon occurs when the Moon
appears 90 degrees or a quarter-circle west
(clockwise 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) Using the Inner Planets Chart, find which
two planets will appear close together in our
sky on Jan. 10, 2015? On Feb. 21, 2015? On
Apr. 22, 2015? For each pair, determine time
of day it will be seen, at dusk or at dawn.
(7) Which planet will be at opposition, visible
all night on February 6, 2015? In which
constellation does it appear? Which bright
star will appear about 11° east of that planet?
(8) In which constellation will the Full Moon
appear on the night of April 3-4, 2015?
Which bright star will appear about 10° east
of the Moon that night? As the Earth rotates
on its axis, the star will appear to follow the
Moon across the sky that night. There will be
a total eclipse of the Moon visible from
western U.S. before dawn on April 4, as the
Moon passes through the Earth’s shadow.
(9) Which four planets will be visible at dusk
(soon after sunset) on April 22, 2015? List the
four planets in order of their positions in the
sky, from western horizon toward eastern
horizon. As the Earth rotates, which two
planets will set almost together, first and
second? Which will set third? Last? Which
bright planet not visible at dusk will rise later
in the evening?
(10) Saturn will be at opposition, visible all
night, about a week after the middle of what
month in 2015? In which constellation will it
appear? Which bright star will appear about
11° east of Saturn?
(11) Near the beginning of which month in
2015 will Venus reach its greatest angular
separation from the Sun in the afternoon and
evening sky? In what phase will Venus
appear then? Observe it through a telescope
in the daytime around then, and with
telescopes and binoculars in following weeks.
(12) Which brilliant planets will form a very
close pair on June 30, 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 telescope field.
Describe their appearances.
(13) What alignment involving Sun, Venus,
and Earth will occur on August 15, 2015?
Notice Venus will be located in the portion of
its orbit plotted as a dotted curve, rather than
solid. During the alignment on Aug. 15,
2015, will 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 passed directly in front of the Sun’s
disk, causing 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? During the weeks
leading up to and following each of these
inferior conjunctions of Venus (June 5, 2012;
Jan. 11, 2014; and August 15, 2015), what
will be the phase of Venus?
(14) Which two planets will appear within
about one degree of each other on October 25
and 26, 2015? When will they be seen, in
dusk or dawn? Which two other planets will
be visible at the same time? Three of these
four planets will fit within a 5° field, forming
a trio, for eight days, Oct. 22-29, 2015.
(15) 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) Rough dates in 2016
when each of the three outer planets will
appear at opposition and be visible all night?
Robert C. Victor was Staff Astronomer at Abrams
Planetarium, Michigan State University. He now
enjoys providing sky watching opportunities for
school children in and around Palm Springs.