All Things Being Equal?

During the Mystery Class activity, you may have noticed that on the equinox many locations do not appear to have exactly twelve hours of equal day and night. Why do you think this is?

Discover!

The answer to this question has two parts. The first part is that the idea of every place having 12 hours of daylight and night on the equinox is not exact. Everyone is said to have "roughly" or "about" 12 hours of daylight on the equinox. This is because day length calculation can vary depending on who is doing the measurement of sunrise or sunset. For instance, before sunrise, there is twilight, and the exact time of sunrise can be observed slightly differently by different people or sources (so can sunset time).

The second part of the answer --the more technical explanation -- has to do with REFRACTION of the earth's atmosphere, which acts like a lens, bending the rays of light from the Sun, especially near the horizon.

So, when you look at the Sun right at sunrise or sunset, and you see the Sun on the horizon, you are actually seeing over the edge of the earth. Technically, the Sun isn't there above the horizon, and you are seeing just its "apparent" position in the sky--just an image. This effect means that at the moments of sunrise and sunset, the Sun is actually below the horizon. This adds extra time at both times of the day. Maybe an extra few minutes to each side. If there were no blanket of air surrounding the earth, the times of sunrise and sunset would coincide perfectly with the calculated values.

Images: Highpoint University and Colorado State University, respectively