Earth's Seasons

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Module 2 Star Gazing
Activity 2 Earths Seasons
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Summary
In this Activity, we will investigate
(a) the Earths orbit around the Sun, (b) the o
rigin of the seasons on Earth, and
(c) the Earths precession.
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The Ecliptic
At the end of the last Activity we introduced the
ecliptic, which is the apparent path of the Sun
across the sky.
The Earth takes one year to make a complete orbit
around the Sun. What else do you know about the
Earths orbit around the Sun besides how long it
takes?
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(a) The Earths orbit around the Sun
You might think that the Earth travels around the
Sun in a circular orbit ...
but actually the shape of the Earths orbit is
an ellipse.
You can think of an ellipse as a squashed
circle(but note that this one is quite
exaggerated!).
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Elliptic orbits can have various shapes, from
squashed, or more technically, orbits of
higheccentricity
to
nearly or completely circular, or more
technically, orbits of low or zero eccentricity.
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A circle is in fact a special case of an ellipse.

• Ellipses are characterised by their eccentricity
  e, which varies from

e 0.99999...
e 0
e ? 0.8
Circles are ellipses with zero eccentricity.
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The Earths orbit is nearly circular, with e
0.0167.
Its average distance from the Sun is 149,597,900
km.
We write large numbers like this in a
mathematical shorthand called scientific
notation, where
149,597,900 km 1.49597900 x 108 km
where the 108 is 1 followed by 8 zeros. This is
the same as multiplying 1.49597900 by 10 eight
times.
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In astronomy, however, we have more convenient
way of representing the average distance from the
Earthto the Sun
its defined as one Astronomical Unit,
where 1 AU 1.495979 x 108 km

1 AU
Well find Astronomical Units (AU) convenient
whenwe compare distances between the Sun and
other planets in our Solar System.
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The small eccentricity of the Earths orbit
(0.0167) means that its distance from the Sun
varies by 0.0167 x 2 x 1.495979 x 108 km, or
about 5.00 x 106 km during the course of a year.
This is a variation of only about 3 in the
overall orbital radius, but represents a distance
of about 400 times the Earths diameter.
For more information about elliptic orbits, click
here.
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(b) The origin of the seasons on Earth

• As we saw in the last Activity, one (Earth) year
  is the time it takes for Earth to make a complete
  orbit around the Sun.

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However, we cant really feel that the Earth is
orbiting the Sun (even though the Earth is
travelling 30 km/s!), and these days not many
people take notice of the Earths orbital
position (that is, people dont take much notice
of the changing of the constellations in the
night sky).
We primarily notice the passing of a year by the
cycle of the seasons.
Other planets have seasons too. Investigating
the reasons for Earths seasons will help us
understand the conditions on other planets also.
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• So what is the cause of the seasons?

Clearly the seasons have something to do with the
Earths orbit around the Sun. Yet many people
are confused about why the Earth has seasons.
Before going on to the next slide, have a think
about it yourself what is the cause of the
seasons?
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• The seasons are caused by the changing distance
  between the Earth and the Sun, and it is warmer
  in summer because the Earth is closer to the Sun
  at summer time.

This is a very common response - and it is true
that the Earth-Sun distance does charge. As we
just saw, the Earths distance from the Sun
varies by about 3 during its orbit.
So could summer occur when the Earth is closest
to the Sun? The problem with this idea is that wh
en its summer in the northern hemisphere, its
winter in the southern hemisphere, and vice
versa. So if this were the correct answer, it
would be the same season in both hemispheres at
the same time - which is not the case.
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The Earths rotational axis is tilted by 23.5
with respect to a line drawn perpendicular to the
plane of the ecliptic.
23.5

The seasons result from this tilt of the Earths
axis of rotation.
This is true not only of the Earth, but all other
planets with tilted rotation axes, as we shall
see.
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The direction of the rotational axis stays
(nearly) fixed in space while the Earth orbits
the Sun and the hemisphere that seems to lean
into the Sun experiences summer, while the
hemisphere that leans away from the Sun
experiences winter.
northern hemisphere leans away from Sun?
winter in NH
summer in NH (leans into Sun)
winter in SH (leans away from Sun)
southern hemisphere leans into Sun? summer in
SH
And six months later the opposite is true
Thus the tilt of the Earths rotation axis
naturally explains why the seasons are opposite
in the northern and southern hemispheres.
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In December, when the southern hemisphere is
tilted towards the Sun, the southern part of the
Earth receives more sunlight and experiences long
summer days.
At the same time, the northern hemisphere is
tilted away from the Sun and receives less
sunlight, experiencing short winter days.
If you live near the Equator, there is not much
difference between the seasons all year round.
equator
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If we take the case when the northern hemisphere
is in summer ...

sunlight
not only does the northern hemisphere receive
more sunlight, but it receives more direct
sunlight because the Sun is higher in the daytime
sky. This helps heat the atmosphere in summer.
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When the Sun is higher in the summer sky, the
sunlight is more concentrated .
than in winter, when the Sun is lower in the
sky, and the sunlight is more diffuse.
Concentrated beamof Summer sunlight
Diffuse, spread-out beam of Winter sunlight
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so for the hemisphere experiencing Summer,
sunlight striking the Earth is more concentrated
and this helps to raise the average temperature.
The reverse is true for the hemisphere
experiencing Winter.
During spring and autumn, the two hemispheres
receive approximately equal amounts of sunlight.
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Lets have a look how it works during the course
of a year
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As weve already mentioned, if you live near the
Equator, you dont really notice the changing
seasons during the course of a year. Generally
you just have two seasons dry and wet!
If you are at the North or South Pole, then also
experience two very long seasons in summer, the
South Pole is leaning towards the Sun and there
is daylight for nearly six months. In winter,
the South Pole is leaning away from the Sun and
it is nighttime for nearly six months.
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If we take the case when the northern hemisphere
is in midsummer ...
then the north pole has continuous daylight
and locations in the northern hemisphere have
long periods of daylight,
sunlight
equator
whereas locations in the southern hemisphere
have long nights.
the south pole is in continuous darkness
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So in summary, the cause of the seasons is the
tilt of the Earths rotational axis, and as a
consequence of this tilt
the Sun is higher in the sky for longer during
summer days (and lower in the sky for a shorter
number of hours on winter days)
and because the Sun in higher in the summer
sky, the heating of the Earths surface
is more direct (whereas the low winter
Suns heating is more diffuse).
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As we will see, whether the rotational axis is
tilted or notdetermines whether other planets
experience seasonstoo.
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(c) The Earths precession
During its yearly orbit around the Sun, the
Earths rotation axis is fixed in space.
However, if we could watch the orientation of the
Earths rotation axis over a very long period of
time (about 26,000 years!), we would see that it
in fact precesses.
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• The precession of the Earth is due to the
  gravitational tug of war on the Earth by the
  Sun and the Moon.

Not to scale!
The Earths rotation creates an equatorial
bulge (meaning the Earth is fatter at the
equator than at the poles).
The Earths tilt means the Sun and Moon are not
aligned with the equator, and both the Sun and
Moon try to pulls the Earths equatorial bulge
closer to it.
The combined pull of the Sun and Moon, along with
the Earths own rotation, result in the observed
precession.
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Over a period of 26,000 years, the Earths
rotational axis precesses through a complete
cycle.
Click here to see an animation of precession.
It is this precession which has gradually shifted
the positions of the constellations in the sky,
and, in particular, the periods of the year
which correspond to each zodiacal constellation.

See the previous Activity, Star Patterns
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Precession also changes the locations at which
seasons occur in the Earths orbit.
The Earth is currently closest to the Sun during
southern summers, but in about 13,000 years it
will occur duringnorthern summers. This may
cause southern summers to become more mild, and
northern winters to become more severe.
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Image Credits

• NASA View of the Mid-Pacific Oceanhttp//nssdc.g
  sfc.nasa.gov/image/planetary/earth/gal_mid-pacific
  .jpg
  

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• Now return to the Module home page, and read
  more about the Earths seasons and precession in
  the Textbook Readings.

Hit the Esc key (escape) to return to the Module
2 Home Page
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Elliptic Orbits

• The Cartesian (i.e (x,y) coordinates) equation
  for an ellipse is given by

where a is the semi-major axis
and b is the semi-minor axis.
If a b, then the ellipse becomes a circle. The
larger a is than b, the more squashed the
ellipse is.
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• We can also write the equation for an ellipse in
  polar coordinates (i.e (r,?) coordinates)

where r is the radius
and ? is the
angle (from the x-axis).
r and ? are measured from the focus of the
ellipse.

The eccentricity e, which is given by
describes how squashed the ellipse is, with
circles having e 0.
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• If we use polar coordinates to plot the ellipse,
  we need to run from ? 0 to ? 2?? (or ? 0
  to 360).

An ellipse, however, will look like a sine wave
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Return to Activity
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