Download Seasons on Other Planets

Astronomy Background • 3.7
Seasons on Other Planets
by Andrew Fraknoi
(Foothill College & ASP)
W
e saw (in background article 3.5) that the
Earth has seasons because a cosmic accident
tilted its axis (the imaginary line around
which it rotates.) Our planet’s axis is not “straight up”,
but at an angle of 23 degrees from vertical. The Earth’s
orbit is nearly circular, so that distance has almost nothing to do with the seasons. The tilt dominates. But what
are the seasons like on other planets?
On other worlds, we can’t assume that conditions are
the same as on Earth. For each planet, we have to ask:
1. Is the axis tilted from the vertical? (The greater the
tilt, the more pronounced the seasons.)
2. How circular is the orbit around the Sun? (If it’s
not circular, then at some points the planet will be
farther from the Sun and a bit colder.) The shape of
a planet’s orbit is defined by its eccentricity. A circle
has zero eccentricity; the larger the eccentricity,
the more a planet’s orbit is an ellipse, rather than a
circle. Significantly eccentric orbits will mean that
the planet is sometimes is quite a bit further from
the Sun than at other times and that the distance
will also contribute to the seasons on that world.
Below is a chart showing tilt and eccentricity for each
planet:
A few specific elaborations:
MARS The seasons are the most complicated on Mars,
because both the tilt of 24° and the changing distance
from the Sun (larger eccentricity) play a role. Mars is
farthest from the Sun when it is winter in the Southern Hemisphere and summer in the North. It’s closest
to the Sun when the South has summer and the North
has winter. So the seasonal differences in the South are
more extreme; they are more mild in the North. (Put
another way, the two effects reinforce each other in
the South, but work against each other in the North.)
SATURN Saturn takes almost 30 years to orbit the Sun,
Tilt and Eccentricity for Each Planet
Planet
Axis Tilt
Eccentricity
Mercury
0o
0.21
Very complicated. The orbit is significantly eccentric, but in addition,
Mercury spins very slowly (59 Earth days). Its year (time to orbit the
Sun) is 88 days. That means it spins three times every 2 Mercury years.
Venus
3o
0.01
No seasons. Atmosphere is very thick. Always hot all over.
Earth
23.5
0.02
Yes, roughly three months for each season
Mars
24o
0.09
Yes, but both tilt and distance play a role (see below). Seasons last
about twice as long as on Earth.
Jupiter
3o
0.05
No
Saturn
o
27
0.06
Yes, about 7.5 Earth years for each season.
Uranus
82o
0.05
Yes, takes 21 Earth years for each season; but seasons are complex
because the planet “spins on its side” (see below)
Neptune
29o
0.01
Yes, each season is about 41 Earth years.
o
Are There Seasons?
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 1
3.7 • Seasons on Other Planets
Astronomy Background
but only about 10 hours to spin. While heat rising
from inside Saturn dominates the inside regions of
the planet, the outer cloud layers do experience seasons. There seems to be a roughly 30-year great storm
cycle, the chemistry of the upper layers changes with
the seasons, and how we see the rings tilted changes
because of the planet’s tilt. The seasonal cycles on Saturn are still a subject of scientific investigation.
© 2010 Andrew Fraknoi. Permission to use for any
non-profit, education purpose is hereby granted.
Published by the Astronomical Society of the Pacific
www.astrosociety.org
URANUS The planet orbits on its side. That is, the axis
around which Uranus rotates is roughly in the same
plane as the planet’s orbit. This leads to strange seasons. At the height of summer for the Northern Hemisphere, for example, the axis points almost directly at
the Sun. This means the Northern side of the planet is
in continuous sunlight, while the Southern side is in
continuous darkness. A floating platform above one
of the poles of Neptune would experience 42 years of
sunlight and then 42 years of darkness.
A diagram showing the tilt of the axis on all the planets plus Pluto:
Uranus
98° (–82°)
Mercury
0°
Earth
23.5°
Venus
177° (–3°)
Mars
24°
Pluto
118° (–62°)
Jupiter
3°
Saturn
27°
The Universe at Your Fingertips • Astronomical Society of the Pacific
Neptune
29°
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