Blue Collar Scientist

Welcome, Mix 103.1 Listeners!

It is often believed that on the equinox, the length of daylight and nighttime are the same. The very term “equinox,” with its similarity to equality, implies it. But in reality, there’s more daylight than nighttime on the date of equinox.

The moment of equinox is when the center of the sun is directly over the equator. If you were standing on the equator, the sun would pass directly overhead. But the sun wouldn’t stay above the horizon for twelve hours, and then set, leading to twelve hours of night. If you had a stopwatch and timed it, you’d find there were several minutes of excess daylight at the equator. And if you timed it in Alaska, the excess would be far greater - more than a third of an hour!

Why would this be?

The biggest factor in messing up this otherwise perfect symmetry is our atmosphere. When looking along the horizon, we are looking through a vast sea of air, which refracts the light from all celestial bodies - including the sun. The result of this refraction is to make everything low to the horizon appear to be higher in the sky than it really is. The effect is so extreme that while the sun is still below the horizon, the atmosphere refracts its image and makes it appear to have already risen. At the horizon, atmospheric refraction bends this image by more than the apparent size of the sun. If we took the atmosphere away the instant that the entire sun became visible over the horizon, the sun would disappear, and it would be a few seconds before it rose a second time.

Of course this effect works in reverse as well. Because the atmosphere makes the sun appear higher in the sky, during sunset the real sun is below the horizon even though the image of the sun is still visible.

How much of a difference this makes depends on your latitude. On the equator, when the sun rises it goes directly up, gaining altitude but remaining over whatever distant mountain or building it rose from behind. So the difference in time between the optical illusion of sunrise and the moment when the actual sun would appear above the horizon if there weren’t an atmosphere is only a few dozen seconds. Similarly, during sunset, the sun plunges straight below the horizon, leading to a similar time difference.

But from more northerly and southerly latitudes, the sun moves to the left or the right during sunrise and sunset. At the latitude of Anchorage, this motion is to the right, and the sun actually moves more to the right than vertically during the course of the day. One consequence of this is that sunrise and sunset are protracted, long-lasting events. When the first bit of the sun appears above the horizon, it does not appear to be in as big a hurry and it doesn’t rise straight up into the sky; it skirts along the horizon, allowing the sunrise to linger for many minutes. This means that the image of the sun appears above the horizon for quite a long while before the “true” sun would appear if there were no atmosphere. And again, the same process works during sunset to further extend the period of daylight.

The end result is that we get far more daylight on the equinox than we do nighttime. Today, sunrise was at 7:56 AM, and sunset will be at 8:19 PM. That’s 23 minutes of excess daylight! For us, here in Anchorage, we had equal daylight and nighttime on Monday (March 17) - when the sun rose at 8:08 AM and set at 8:08 PM.