## Friday 5 February 2016

59) “Quoting Gabrielle Henriet, “The theory of the rotation of the earth may once and for all be definitely disposed of as impracticable by pointing out the following inadvertence. It is said that the rotation takes twenty-four hours and that its speed is uniform, in which case, necessarily, days and nights should have an identical duration of twelve hours each all the year round. The sun should invariably rise in the morning and set in the evening at the same hours, with the result that it would be the equinox every day from the 1st of January to the 31st of December. One should stop and reflect on this before saying that the earth has a movement of rotation. How does the system of gravitation account for the seasonal variations in the lengths of days and nights if the earth rotates at a uniform speed in twenty-four hours!?”. How does the system of gravitation account for the seasonal variations in the lengths of days and nights if the earth rotates at a uniform speed in twenty-four hours!?”

Dubay uses another diagram that is nonsense on its own, because it leaves out the axial tilt.

Amusingly, Mr Dubay stole this picture from this website which explains how the seasons work.
He’s obviously doubly dishonest, because he pretended not to understand this back at point  49.

The one Mr Dubay cherrypicks is for one day of the year, Figure E: Autumnal (Fall) Equinox/Vernal (Spring) Equinox

He really, really didn’t want you to see the other diagrams, because they squash this silly point instantly. Here is a good video explaining why: https://youtu.be/Y4zFX8f2in8

This is the version in the Northern hemisphere summer:

See how the sun will dip below the horizon earlier in the south, while the northern day is longer, because the sun "peeps over" the northern curve of the earth for longer in the northern summer.

And this is the southern hemisphere summer.

If you still don't quite see it, try experimenting with a ball and a table lamp. Tilt the "north" end of the ball about 23 degrees away from the lamp  and  watch how a larger area of the "south end is lit up as you rotate the ball.

Daimonie explains further:

“I made you an image to explain, using a globe I grabbed from google. It's not much different from the image we saw in #56, but shows more details we want to focus on. Mind you, it is a little bit skewed because the globe shows the pole. Still, it suffices.

 A picture I made to explain the seasons and day/night length.

So, let us first look at the seasons. As you can see I drew in the axis of rotation and the equator. Quite clearly, when you compare left/right, the position of the word 'the sun' is different with regards to the equator. In the left image, it is below the equator, and in the right image it is above the equator. That explains why there are seasons, for instance because of the Poynting theorem we discussed at one point, that related the inclination angle of light to the (max) intensity absorbed. Wikipedia contains a rather nice animation which I recommend.

As for the length of day\night, this too can be readily seen in the picture. Draw the line that goes straight through the sun. This line intersects the globe. You can see that the darkened areas vary with latitude; and that explains the difference in day and night lengths. Yes, it's actually that simple.” (Daimonie)