## Thursday, April 8, 2010

### Mystery Class Math

A major part of this project is calculating photoperiod, that is the amount of daylight at the ten mystery class locations.

We've been doing this by figuring out how much daylight was gained or lost at sunrise and sunset and then adding or subtracting from the photoperiod for the previous week.

While doing MEP with my son, I realized that it was simple enough to do this calculation in a way that extends my children's understanding of place value and regrouping, as well as introduce bases other than base 10.

Many of you are probably already aware of how to make this calculation, but if not I hope you find this as easy and helpful as I did.

Let's say sunrise is 07:26 and sunset is 20:10.  We have two place values: hours and minutes.  We can only put the numbers 0 to 23 in the hours place.  Our sunrise has 7 in the hours place while sunset has 20 in the hours place.

In the minutes place we can only put the numbers 0 to 59.  Sunrise has 26 in the minutes place while sunset has 10.  Now set this up as you would any other vertical subtraction to calculate the photoperiod:

20  10
-07  26

You cannot subtract 26 from 10 so you borrow from the 20 hours, making it 19 hours.  That one hour becomes 60 minutes in the minutes place, so the 10 becomes 70 (60 + 10.)  Now we have:

19  70
-07  26

70 - 26 = 44 minutes, 19 - 7 = 12 hours.  The photoperiod is 12 hours and 44 minutes.