Cloudy Night, Can We Still Observe Anything?

With clouds covering most of sky locally, it might seem like there's nothing for aspiring astronomers to observe. But there's still one interesting observation we can make.

In our daily lives we have a general sense of the lengthening days in spring and the progressively longer nights in the fall. The direct causes of this are a deeper discussion for another time, but for now we'll simplify to say that as the Earth revolves around the Sun every 365 days, it spins in the same direction which is slightly lower than straight up and down. Imagine spinning a basketball on your finger, but it was leaning and continued spinning on a slant.

Because the Earth is tipped relative to the plane of its orbit, as it moves around the Sun it causes one side to receive more exposure when on one side, and less when on the opposite side. The key points are the solstices, when the Sun ceases to climb higher each day and begins to turn around and appear slightly lower each day. This is the Summer Solstice (solstice is Latin for "sun" and "still", when it seems to stop for a day as it changes from climbing to descending - just like throwing a ball straight up, at the turnaround at the top it seems to freeze instantaneously). And the converse is true six months later at the Winter Solstice, when it switches from descending to climbing.

The halfway points between the Summer and Winter solstices are called "equinoxes," which mark the first day of Spring and Fall (called the Vernal Equinox and Autumnal Equinox respectively). This happened recently, on March 19, 2020.

On the equinox, the night is exactly (ok, nearly exactly but due to the eccentricity of Earth's elliptical orbit it actually happens out of phase by two weeks, but for the sake of simplicity...) as long as the day, which is where we get the name from Latin "equi" = equal and "nox" = night.

Here on April 2nd, we're just passed this point, so the daytime is slightly longer than night, and the ratio of day to night will continue until the Summer Solstice, when it's at the maximum daylight to night ratio.

This graph shows the Sun's elevation in the sky as it climbs to its peak each day, notice how it steadily increases each day. The bands represent day and night, notice how they're relatively close now, but as we check in closer to the Summer Solstice you'll see the day to night ratio has changed significantly.

Plot of the Sun’s daily incremental elevation gain

Using nothing more than a clock you'll be able to track the sunset times, even nights like this when the clouds prevent much else in the way of observing. A good exercise is to make a mental note of the sunset time and compare each week, you'll begin to see that the amount by which it change is most drastic near the equinox and it will begin to slow as we near the Summer Solstice.

You can experiment with this effect in the SkySafari app for Android/iOS if you want to simulate different times of the year.

Clear skies!