Cutting Grass on the Moon

I cut the grass today.

This is nothing exceptional really. I do it every week or two. But today, I found it rather challenging. For some reason, it took decidedly longer than usual.

I suspect that it was due to me cutting areas I'd already done because I was having a great deal of trouble telling where I'd already cut. Usually I do this by looking to see where the grass was suddenly an inch shorter, or by looking for the depressions caused by the wheels on the grass.

Today, both were exceptionally hard to find.

So what was different? Certainly it wasn't he grass. It's the same yard I've mowed over and over. The lawnmower didn't suddenly get lighter and not leave any impressions.

There was only one thing that changed: The time of day. Today, I cut the grass near noon. Usually I cut the grass in the early morning. But aside from it being nice and cool in the morning, how could the time of day possibly make a difference?

To answer this question I had to think about the moon.

If you've ever looked at the moon through a telescope, you may have noticed that there's certain times that it looks better than others. Generally, the best times are near the quarter moons (1st and 3rd) where the moon is half-lit from our vantage point.

The reason that these times are especially good is that the sun is striking the visible surface at a shallow angle. As a result, any topological changes that are above a nearby surface will cause long, sharp shadows. This is especially important in craters where crater rims are often raised slightly above the nearby terrain and can cast shadows into the bowl of the crater. Meanwhile, the opposite side of the crater will receive sunlight and be well lit.

This alternation of light and dark provides a sharp contrast to help define the features and make them stand out. When the moon is in the full phase, the sunlight is beating straight down making the shadows small and contrast almost non-existent.

The same thing happens when I'm cutting the grass. At noon, the Sun is at it's highest point making even the minor shadows that something as diffuse as grass can cast rather minimal. In the early morning hours, the sun is sill lower on the horizon, but strikes the grass at a shallow angle allowing for greater contrast. The grass I haven't cut can cast a shadow on that I have. Furthermore, the wheels will create additional low lying areas that can also pick up shadows and stand out.

With this in mind, I think I'll avoid cutting the grass near noon in the future. The only problem with that is, now I'll have to wake up early. And us astronomers are naturally nocturnal.