Question about sunset and dusk - prefer more science minded responses...?

Question

I manage a drive-in movie theatre so I am very aware of what time it's dusk during the summer season. Over the entire summer, dusk time changed by no more than a few minutes from week to week. However, a few weeks ago I was at the drive-in on Sunday night at about 8:45 and it was just barely dark enough to start the movie. By the following Friday (5 days later) it was suddenly dark enough by about 8:20.

Can anyone shed some light on why dusk time would change so drastically in just a few days when it normally only changes by 5 minutes over a week. Could it has something to do with the sun's position relative to a distant mountain range or something similar?

For what it's worth, I live in northern California about 3 hours north of the Bay Area.

Answer 1

Are you familiar with the sine function?
If you are, this will be a lot easier to explain (see attachment for a sine function plot, the red curve)
At zero and Pi, the value of the sine function is zero, lest say these represent how much longer daylight as compared to night time; the same length. This would be the equinox (March 21 or so, and September 22 or so; respectively the beginning of spring and fall). Also note that the highest and lowest point of the curve (at Pi/2 and 3 Pi/2) would be the summer solstice (longest day, June 21) and winter solstice (shortest day, December 21st).

Now, imagine deviating just a few day from the 21st of June, how much the day length change? Very little. Now, look around the time when the days are about as long as the night, i.e. late September. Notice how steep the curve is? That is when the duration of the day changes the most, from day to day.

And that's all there is to it.

Answer 2

The picture below might shed some light on the issue. It's an analemma, or a picture of where the sun is at noon during the year. The lowest point is the winter solstice, the highest is the summer solstice, and the midpoint where it meets are the equinoxes. As you can see, and already know from what you've said here, the sun reaches it's highest point in the summer and never rises as high in the winter.

Not sure how much this helps, but you can get an idea of how far the sun moves in the sky at different times of the year. The suns in the picture are taken ~10 days apart at noon. We should currently be on the lower side of the lower loop, heading for the lowest point.

http://www.definity-systems.net/~apw/astro/images/analemma_vr.jpg

Answer 3

the earths position on its axis is constant so that the spin of the earth changes the position of northern calif. gradually bringing it further from the suns direct line. you dont gain exactly the same amount of time each day through the year.

The angles change so that you might gain just over half a minute just after the summer solstice and before the winter solstice, but it increases to a maximum daily increase of a minute and a half or more nearer to the equinox in spring and fall. Its a cumulative difference that happens on an uneven but predictable scale.

think of the sunset itself. you dont notice the movement of the sun in the sky from one hour to the next at two oclock in the afternoon. How many can tell time by the sun?

but you do notice its movement near sunset. The effect appears much faster as the light disappears over the curvature of the earth.

it is the same effect

Answer 4

I've noticed a similar change in the arrival of darkness on the coastal plain on the east coast. I've recently [minutes ago] learned that tilt of the earth's axis and the slightly elliptical nature of the earth's orbit around the sun cause changes in the times of sunset (and sunrise). I found a thorough answer at Cornell.