What the seasons are NOT due to…
NOT
the ellipticity of the Earth’s orbit:
Our
ellipticity is very small, and we are closest to the Sun in January.
You can
most easily see this by remembering that at the same time it is Winter here, it
is Summer in the Southern Hemisphere.
NOT
the fact that one pole is closer to the Sun because the Earth’s
axis
is tilted:
The size of
the Earth is miniscule compared with its distance to the Sun, so it
doesn’t matter how things are oriented, they are all about the same distance
from the Sun.
Note: Summer is not warmest at the solstice
(June) because it takes a while for the seasonal effects to really kick in,
so we tend to be hottest a couple of months later (and coldest a couple of
months after
the winter solstice).