“What’s Up”
ASTRONOMERS EXPLAIN WHY WE HAVE SEASONS
July 10 - Mid August
North and south of the tropics the earth
experiences seasons. In Canada that
means cold winters, warm summers and
everything in between. But why?
If you make a drawing of the
earth’s orbit around the sun, the sheet
of paper is what we call the plane of
the earth orbit. If you could look at
the paper edge on you would see that
the earth’s axis of rotation is tilted by
23.5 degrees from the vertical.
What do we observe on earth? The sun rises in the east and sets in the
west. Really it is the earth that is rotating from west to east but the sun
appears to move. In winter we say the sun is low in the sky, and we have
shorter days. In summer the sun is higher in the sky and we have
longer days. In the northern hemisphere the sun reaches its highest
point in the sky on June 20 or 21 and this is the longest day, the
summer solstice. On December 21 or 22 the sun is lowest in the
sky and this is the shortest day of the year, the winter solstice.
Naturally it is the reverse in the southern hemisphere.
S
Illustrated by: Shawn Martin
After the shortest day in December the sun appears to get higher in the
sky each day for us in Canada. There is always some point on earth where
the sun is directly overhead at some time in the year but it cannot be
overhead anywhere north of the Tropic of Cancer at latitude 23.5 degrees or
south of the Tropic of Capricorn at 23.5 degrees south of the equator. Note
the latitudes of the two tropics and compare with the tilt of the earth’s axis,
they are the same.
The instant when the sun is directly over the equator on March 20 or 21
the days and nights are approximately equal and we say we are at the spring
equinox. After that the sun is north of the equator giving us spring, then
summer. The days get longer until the longest day in June. After the
Solstice the days get shorter and the sun moves southward. When it is again
directly overhead at the equator we have the fall equinox on September 22
or 23. After that the days continue to get shorter until the sun reaches its
furthest southerly point and the shortest day at the Winter Solstice.
But why do we have seasons? The amount of
energy earth gets from the sun is relatively
constant from day to day. When you shine a
flashlight on a piece of paper there is a round
area illuminated. But if the flashlight is tilted,
the illuminated area becomes a larger elliptical
area. In both cases the paper is getting the same Illustrated by: Shawn Martin
total amount of light but in the second case the light is spread over a larger
area giving less light for each square centimeter. Similarly when the earth’s
north pole is tilted away from the sun, the sun’s energy is spread over a
larger area and it is cooler (winter) in the north but summer in the southern
hemisphere. This explains the seasons as long as you remember that things
do not change temperature immediately when heated. When the sun is
higher in the sky in spring it takes time for the rock, and especially the
water, to heat up. So when we say it is spring because the sun is north of the
equator there is still a lot of heating to be done and that takes time.
Seasons, as we know them, do not occur in the tropics because the sun is
always high in the sky in tropical areas.
The tilt of the earth’s axis also
explains why, in summer, people living
above the Arctic Circle experience 24
hours of daylight but in winter 24 hours
of darkness. In summer the north is
tilted towards the sun and sunlight
covers the Arctic. In winter the sun is
always below the horizon and there is
24 hours of darkness.
Illustrated by: Shawn Martin
Now imagine the earth moving
in its orbit around the sun with its
axis of rotation always at the same
angle of 23.5 degrees. In the
drawing, when earth’s north pole is
tilted away from the sun, the south
Illustrated by: Shawn Martin pole must be tilted towards the sun.
At this point it is winter in the northern hemisphere and summer in the
southern hemisphere. Six months later the north pole is tilted towards the
sun and we have summer and the south pole is tilted away and people in the
southern hemisphere have winter.
er
Illustrated by: Shawn Martin
Fred Smith RASC, St. John’s Centre
ACTIVITIES
1. What is solar energy? Look in The Telegram for an
advertisement offering to install materials and equipment for solar
energy.
2. Look in The Telegram to find how many ways the sun
contributes to our well being and life style.
For more activities go to
www.thetelegram.com and click on
Brought to you by
W
E
Point this
astronomical chart
toward North and
match the stars with
those in the real sky.
Planets
S
Viewable in a pair of Binoculars or small telescope
Mercury - is hidden in the glow of dawn.
Venus - (magnitude –4.2) in the east during dawn.
Mars - (magnitude +1.1) in the east during dawn.
Jupiter - (magnitude –2.7, in Capricornus) rises around 10 or 11 p.m.
Saturn - (magnitude +1.0, in Leo) glows in the west at dusk.
Uranus - (magnitude 5.8, in Pisces), is high in the southeast before dawn.
Neptune - (magnitude 7.9, in Capricornus) remains only 0.6° from Jupiter.
Pluto (dwarf planet) - (14th magnitude, in northwestern Sagittarius).
Moon
Waxing
First Quarter
Full
July 26
July 28
August 5
Last Quarter Waning
July 15
August 13
New
July 17
July 22
Meteor Shower
The Perseids meteor shower is one of the largest and best, it occurs annually every August
on or near the evenings of the 11-13th. They are the result of the Earth as it passes
through a cloud of dust and particles left behind from the periodic comet Swift-Tuttle.
Called the Perseids meteor
shower because of their visual
location (Radiant), in the
constellation Perseus. (see
illustration). To view this
meteor shower look toward the
constellation Perseus on the
evening of August 11/12 to the
North East at At approximately
12 a.m. midnight. A waxing
gibbous moon will wash out the
night sky, but once it sets the
skies will darken providing
greater contrast for detecting
more Perseid meteors.
You can contact the Royal Astronomical Society of Canada, St. John’s
Centre at www.rasc.ca/stjohns/
WA R N I N G ! “ W h e n u s i n g a t e l e s c o p e o r b i n o c u l a r s , a l w a y s b e s u r e
NEVER TO LOOK AT THE SUN! This can cause serious and permanent eye
damage. To be safe, always make sure the Sun is fully set below the horizon
before going outside with your telescope or binoculars.”
pa
news
pe r s. Open m
in
ds
n
pe
Newspaper In Education
THE TELEGRAM
0-3729494
The earth rotates on its axis once a
day and revolves around the sun once
a year. The axis of rotation runs
through earth’s north and south poles
and determines true north and true
south.
m
N
art
Ch
r
ta
O
How to draw an ellipse: An ellipse is a
flattened circle. A circle has a centre but an
ellipse has two centres. You can draw an
ellipse by making a straight line on a piece of
paper and then, with the paper placed on a
piece of wood or foam core so you will not
mark the table, push two thumbtacks into
points on the line. Now take a piece of string,
tie a knot to join the ends and place the loop
over the thumbtacks. Put your pencil into the
Illustrated by: Shawn Martin
loop and pull tight. As you move the pencil an
ellipse forms. Experiment with different points and lengths of string. Note,
as you move the thumbtacks closer together the ellipse becomes more
circular.
Su
m
The earth travels around the sun in its
orbit and the sun is our main source of
energy. Back in the 1600s Johannes Kepler
showed that orbits are not circles but ellipses.
This means we are sometimes closer to the sun
and other time farther away. Do we have summer
when the earth is nearer to the sun? No, many people believe that but it is
not correct. Once we developed the ability to measure the distance from us
to the sun we realized that the difference between our closest point (the
perihelion) and our furthest point from the sun (the aphelion) is not great
enough to produce the seasons. Also the earth is closest to the sun in
January which is winter in the north and furthest in July when it is summer
in Canada. When you see a drawing of the earth’s orbit it looks like a circle;
it is not very elliptical.
Shawn Martin Observing Director RASC, St. John’s Centre