## Tuesday, June 26, 2012

### How much daylight is left?

In yesterday's post, we learned about the angle, the foundation measurement tool for observing things in the sky. Today you'll use those tools, and your hand, to get a quick estimate of the amount of daylight left.

First, we should figure out how quickly the sun moves across the sky. Because the earth rotates once every 24 hours, the sun appears to move across the sky once every 24 hours. To be more specific:
$\frac {360^{\circ}}{24 hours} = 15^{\circ}/hour$

Since the width of your hand spans approximately 10-15 degrees (mine is relatively "fat" and covers 15), you can use it to approximate how far the sun will move in an hour. So, what I've done on many occasions is counted how many hands up from the horizon the sun is, and approximated how long till sunset. Since you have 4 fingers, they make decent 15 minute approximations.

A few notes -- first, the sun does not travel straight down, but at an angle towards its final resting place. In the Northern Hemisphere (specifically North of the Tropic of Cancer line) where I'm guessing any of you readers are from, the sun will move further north as it sets, so you might need to tilt your hand somewhat to accommodate.

Secondly, it doesn't become instantly dark once the sun sets, but there is plenty of twilight to help you. I typically figure on an additional hour of twilight before it gets too buggy or dark to want to be outside.

If we know the width of the sun, we can calculate how long sunset will take, from the moment the sun first touches the horizon till it dips behind the horizon. The "width" of the sun, and the moon for that matter, is about 1/2 degree. So once the edge of the sun touches the horizon, you'll have:$\frac {\tfrac{1}{2}^{\circ}}{ } \frac{1 hour}{15^{\circ}} \frac{60 min}{hour}= 2 min$ to enjoy the sunset before its gone.

You can also use this fact if your clock on your camera ever goes bad to tell when a picture was taken. In photoshop (who am I kidding... i just used MsPaint, I can't afford photoshop) I projected where the sun was going to travel, and more importantly how many "suns" were left in the sky. Since five suns were left, this picture was taken approximately 10 minutes before "sunset" which you can look up for any particular place and day.