Hi.
I'm Dr. Olathe MacIntyre.
I'm a scientist here at Science North.
I'm going to use this model to demonstrate
the phases of the moon, eclipses and seasons for you.
If the Earth were the size of this quarter,
the moon would be the size of this dime.
And they would be a metre apart.
So, my moon is way too small
and I'm going to hold it way too close to Earth.
Same for the sun, but it works.
My moon is light on one side,
and dark on the other side.
Now, half of the moon is always lit by the sun
but it isn't always the same half.
The moon spins on its axis,
just like the Earth does.
As it orbits around the Earth,
it spins just once on its axis
so we always see the same face of the moon.
So when the moon is aligned with the sun and the Earth,
it's a new moon.
This new moon rises and sets with the sun
so it's in the daytime sky
but we don't see it
because all of the sunlight it's reflecting
is directed away from the Earth.
As the moon continues in its orbit around the Earth,
it begins to wax.
From Earth, we begin to see a crescent
of the lit side of the moon
and the moon is visible at some times during the day.
A quarter of the way through its orbit,
we see a half moon,
which we call the first quarter.
Halfway through the orbit, we see a full moon.
The moon then begins to wane.
3/4 of the way through the orbit,
we're back to a half moon.
We call this the third quarter.
The moon then returns to the new moon position.
So, how come we don't have a solar eclipse
every new moon
and a lunar eclipse every full moon?
Well, that's because
the orbit of the moon
is inclined to the plane that the Earth orbits
around the sun
which we call the ecliptic.
So, every time the moon orbits around the Earth,
it crosses the ecliptic twice,
but we only get an eclipse
if the moon happens to be crossing the ecliptic
during the new moon
or coming up and crossing it during a full moon.
Now the moon takes 29.5 days to orbit the Earth,
which is about a month.
And, in fact, months are based on the moon cycle.
So how come our calendar months
aren't in synch with the phases of the moon?
Well, that's because it takes 11 days longer
for the Earth to orbit around the sun
as it takes for the moon
to orbit around the Earth 12 times.
So, in order for our calendar months,
our 12 calendar months
to synchronize with the seasons,
we've added extra days to the moon cycles.
So, what causes the seasons?
Well, that's the tilt of the Earth's axis.
So you can see the Earth has an axis of spin
that's tilted about 23 degrees
from that plane of the ecliptic that we talked about.
So when the Earth is like this
and the northern hemisphere is tilted
directly towards the sun--
Let me move this over
so we can go all the way around our sun--
...we in the northern hemisphere are experiencing summer.
And actually, in June,
we experience the Summer Solstice,
which is the longest day of the year and the shortest night
and we get the most direct sunlight.
Three months later,
in September,
the Earth isn't tilted towards or away from the sun,
and we experience the Fall Equinox,
which has equal day and night, about.
Six months later,
during December,
we experience the Winter Solstice.
So here we have the longest night of the year
and the shortest day of the year,
and we get the least direct sunlight.
Nine months later, it's March,
and experience the Spring Equinox.
After 12 months,
the season--
the cycle of the seasons is complete.
So I remember how to move this model around
by using the right-hand rule.
So, if I think of the North Pole of the Earth as up,
and I stick my thumb in that direction,
the direction my fingers curl is the direction
that the moon spins on its axis,
that the moon orbits around the Earth,
that the Earth spins on its axis,
and the direction it orbits around the sun.
I hope that helps.