Bob Doyle, Columnist
Clouds, trees and hills
Every day you can read when the sun rises and sets in the Cumberland Times-News weather page. For example, if the local sunrise is at 6 a.m. and sunset is 8:22 p.m., then the sun is above the horizon for 14 hours and 22 minutes. (The trick in determining daily sunlight is to convert sunset p.m. time to a.m. time by adding 12. Then 8:22 p.m. becomes 20:22 a.m. So the amount of sunlight will be 20:22 — 6 = 14:22 or 14 hours and 22 minutes.)
But on the average, clouds cover half of our planet, so the amount of sunlight on an average day is reduced by a factor of two. If we divide that amount by two due to typical cloudiness, then our average daily direct sunlight at this time of the year is about 7 hours.
This average amount of sunlight can be further whittled down by the presence of trees, your garage, barn, your neighbor’s house, etc. So in mid May each person’s average sunlight per day then can vary from 7 to nearly 0 hours (the latter if your dwelling is surrounded by trees).
If you can avoid trees and other obstructions, then finding the amount of sunlight per day depends on two things, the time of year and your local horizon. The sunrise and sunsets times listed on the weather page assume a flat horizon. These times would be useful if we lived in a flat area, but this area is dominated by the Appalachian Mountains.
Locally, we have hills which tower hundreds of feet above the valleys where most of us live. Along your local horizon, the hills and their accompanying tree growth will cut back on the hours of direct sunlight if they appear towards the east and west. If hills stand towards your eastern horizon, your first sighting of the sun will be delayed. If hills are near your western horizon, the sun’s last rays will be seen well before the sunset listed on the weather page.
To measure the hill’s height, make a fist, stretch out your arm and rotate your wrist so your thumb is on top and small finger is at the bottom; your hand (from thumb to small finger) is about 10 degrees high. Make a fist with the second hand and place it on top of your first fist to make a vertical angle of 20 degrees. So with your hands you can go outside and measure the angular height of your local hills. Take care to place your first fist so its bottom is horizontal (where a flat horizon would be).
If you have a hill in the east that is about 10 degrees high (one fist) then you will lose nearly an hour of sunlight each morning. (The sun will not clear the hill for about an hour after sunrise.) Likewise, a hill in the west that’s 10 degrees high will cause the sun to disappear about an hour earlier than the actual sunset time. So if you have 10 degree hills in both east and west directions, your sunlight hours will drop by nearly 2 hours each day. At this time of the year, your average sunlight hours each day will drop to 6 hours. (6 hours is half of the 12 hours of direct sunlight.)
If you measure your east hill to be 20 degrees high (two fists), then the sun will clear the hill nearly 2 hours after sunrise. In the same way, a west hill that’s 20 degrees high will have the sun drop behind the 20 degree hill about two hours before sunset.
Here are duration of daily sunlight for each month on the 21st: January, 9 hours and 40 minutes; February, 10 hours and 50 minutes; March, 12 hours and 10 minutes; April, 13 hours and 20 minutes; May, 14 hours and 20 minutes; June, 15 hours; July, 14 hours and 20 minutes; August, 13 hours and 20 minutes; September, 12 hours and 10 minutes; October, 10 hours and 50 minutes; November, 9 hours and 40 minutes; and December, 9 hours and 20 minutes
The above times are for a flat horizon so to get your daylight hours, subtract up to four hours depending on the hills on your horizon. Then divide by two to get the average duration of daily sunlight to allow for cloudy days.
Last spring shows, moon and Mars
Our spring planetarium programs conclude today at 4 p.m. and 7 p.m. Our May presentation is “Report on Planet Earth” featuring a review of our spring and summer evening skies and the main feature. “Report on Planet Earth” covers the five key Earth numbers, leading Earth myths and the five processes that make our planet an abode of many life forms. Following our 45-minute program, there will be tours of the Compton Exploratorium, featuring a marvelous display of preserved mammals from five continents.
The Planetarium will have occasional programs during the summer months; we will resume our free Sunday public schedule on Sept. 7. The Tawes Hall Planetarium is in mid campus, near the Lane Student Center, the Performing Arts Center and the Clock Tower. (Call (301) 687-4270 for road directions.)
Tomorrow evening, the moon is full, appearing to the right of the Scorpion’s claws. In the following three nights, the moon will rise about an hour later, becoming a morning object by late week.
From Thursday to Saturday evenings, the planet Mars will appear close to the Beehive star cluster of Cancer. To find Mars, look in the west for a pair of bright stars at about the same height. These are the stars Pollux and Castor of Gemini. Mars appears above and to the left, shining steadily. Binoculars will be needed to see the Beehive star cluster.
Bob Doyle invites comments and questions from readers; his email isrdoyle@frostburg.edu.
