THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2458794.16
2019-2020: L
"10,769 days until the 2049 transit of Mercury."
THE DAILY ASTRONOMER
Tuesday, November 12, 2019
Solar Wattage
Try this: lie back, close your eyes and realize that less than 100
million miles away, a furiously hot star is emitting prodigious amounts of
energy into space. It has been doing so for billions of years and will
continue to do so for billions of years in the future. The same star that
shone like molten gold as it set on a sultry Jurassic jungle now casts a
luster onto New England's window frost and will someday recede into
invisibility from the perspective of some future intergalactic spacecraft.
We mention the Sun because the other day a patron approached the console
and asked a brilliant question.
*"How do we know how much power the Sun generates if we're so far away
from it?" *
We can know the Sun's total power output merely by knowing its distance
from us and the value of the solar constant: the solar energy received at
Earth's upper atmosphere per square meter. While this value does vary
slightly due to both solar activity and Earth's constantly changing
distance from the Sun, the average solar constant value is 1.362 kW/m²
(1.362 kiloWatts per square meter)
[image: 1*sLK72aELmoAyTyd4-Z3t3w.jpeg]
*Stellar Power!* If we could capture and utilize the Sun's energy
output for one second, we could fulfill the world's electricity demand
for the next half million years. Each second, Sol emits 3.84
yottaWatts (384,600,000,000,000,000,000,000,000 W) of energy.
Yet, how could we have determined this impressive value since
we're so far away from our star?
We next assume that the Sun produces energy uniformly in all directions.
Now, let's imagine a thin-membraned sphere centered on the Sun and located
at Earth's distance. We know that every square meter of that sphere will
receive the same energy Earth does. (If the Sun decided to be difficult
and emit radiation at varying amounts depending on its latitude and
rotation angle, this calculation would incinerate neurons.) If we can
calculate the surface area of that sphere, we can know the entire amount of
energy the sphere receives. By extension, we would know the amount emitted
by the Sun. The absorption between the Sun and Earth is negligible.
[image: sphere3.jpg]
Fortunately, geometry enables us to measure this value. The surface area
of a sphere is 4πR², where R = sphere's radius, which is Earth's distance
from the Sun. By calculating this value and multiplying the number of
square meters by the solar constant, we can yield that horrendously
large 384,600,000,000,000,000,000,000,000
W value for our Sun.
Even though we're far from the Sun, we can certainly know how much energy
it generates. Comfortingly, we also know that it contains sufficient
quantities of core hydrogen to sustain its thermonuclear fusion reactions
for billions of years more.
__________________
From the catacombs of infinite knowledge:
*What is Watt?*
A Watt, named for Scottiish physicist and inventor James Watt (1736-1818),
is a power unit equal to 1 Joule per second. A Joule, named for English
physicist James Prescott Joule (1818-1889), is equal to the work done by
one Newton over one meter distance.
__________________
To subscribe or unsubscribe from the "Daily Astronomer"
http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER