Thursday, 8 December 2016

Apparent offset of the winter solstice

simple picture of seasonal daylight length
Apparent offset of the winter solstice.

There are many posts about this on the Internet, but

a. Many are confusing, complex or both;

b. Many people who haven't a clue still hurl derision at those who have;

Hence this brief and simple depiction:

Although the winter solstice is within a day or two of the shortest day (amount of daylight), the darkest morning is around December the 10th, and the darkest evening around January the 2nd.

The variation in daylight is due to the axis of rotation of the Earth being tilted relative to the axis of its travel around the sun.

But the exact time of sunrise and sunset depends on two factors, both of which affect the appearance of the sun on the horizon:

1 the rotation of the Earth on its axis,

2. the position of the Earth on its annual journey around the sun.

Number 2 is not uniform, as one might expect in a circular orbit, because the orbit isn't circular. It is elliptical.

As the Earth moves away from the Sun it slows down, as it moves nearer it speeds. For the scientifically minded, this is conservation of angular momentum. It is also conservation of energy (kinetic energy becoming potential energy as the Earth moves further from the Sun). The beauty of maths (and theoretical physics), and the reason many people, including myself, find them fascinating, is that all approaches lead to the same answer.


The good news:

If you live in the Northern hemisphere, in 3 days time the mornings will be getting lighter! Winter Blues are no use, except possibly as an excuse for playing blues guitar :)

tags:

blues, daybreak, mathematics, physics, popular science, winter,