[ TERC VHS-1 Sun Photometer ] [ Manual TOC ]
Every Sun photometer measurement must be accompanied by a
measurement of the Sun's angle above the horizon. You can measure
this angle using TERC VHS-1 software and a computer. Or You can
measure it directly using one of the methods described here.
Why is it important to know the Sun's angle? As the Earth
rotates, the Sun appears to move across the sky. This means that
light from the Sun travels through less atmosphere when the Sun
is high in the sky than when it is low in the sky.
When the Sun is straight overhead (as it is twice each year
if you live in Hawaii), its rays travel through one thickness of
atmosphere or one air mass (m). When the Sun is closer to the
horizon, its rays pass through more atmosphere than when the Sun
is straight overhead. Thus m increases as the Sun moves closer to
the horizon. The m for each of your measurements is 1 divided by
the sine of the Sun's angle (<) above the horizon or m = 1/sun<.
You will need to know the angle of the sun above the horizon
to compute the air mass. You can determine this angle using the
shadow method, a digital level, a spreadsheet formula or by using
various software programs. The TERC VHS-1 spreadsheet can assist you in your calculations, and can be downloaded from: ftp://ftp.concord.org/pub/haze/vhs1-download/If you measure the angle of the Sun
directly, be sure to do so immediately after making the
observations. In any case, it's very important to record the date
and the exact time of your observations. The time information
will permit you or others calculate the Sun's angle using a
computer spreadsheet at a later date should you or they elect to
You can measure the angle of the Sun to within about 1
degree or so by using the TERC VHS-1 as a simple sun dial. You'll
need a bubble level vial (also known as a spirit level) like
those used in a carpenter's level. Follow these steps:
An inclinometer is a protractor with a weighted needle or
dial. When the base of the inclinometer is level, its indicating
needle or dial points straight up (90 degrees). When the
inclinometer is tilted, the needle or dial remains stable, thus
indicating the degrees of tilt. SmartLevel and other digital
carpenter's levels can also be used to find the Sun's angle.
Since there are many different kinds of inclinometers and
digital levels, we can't provide detailed instructions on how to
use each of them. One method is to attach a straight hollow rod
along the base of the level. A small piece of paper should be
mounted near and in line with the end of the rod away from the
Sun. The base of the inclinometer or level is tilted toward the
Sun until a beam of sunlight emerges from the end of the rod and
forms a bright circle on the paper. The instrument is pointed
directly at the Sun, and the angle on the dial or readout
indicates the Sun's angle above the horizon.
Digital levels work best and are more accurate, but they are
more expensive. Since the pointer or dial of a mechanical
inclinometer sometimes sticks, it might be necessary to tap the
instrument when it is at the desired angle.
The TERC VHS-1 spreadsheet provides the Sun's angle and the
air mass for any location on any day and time. Various computer
programs are available that give the angle of the sun for any
time on any day of the year for any location on Earth. Examples
of such programs include AstroCalcTM and AstroCalc PlusTM (Zephyr
Services, 1900 Murray Avenue, Pittsburgh, Pennsylvania 15217
USA). SunTimes, also available from Zephyr Services, gives the
angle of the Sun at solar noon and at 30-minute intervals for any
day and any location you specify.
Solar noon for your location is not necessarily when your
watch shows 12:00, especially if daylight savings time is in
effect. Complete details about how to determine the local time
for your location are given in various books and magazines about
astronomy and sundial making.
Briefly, the Earth rotates 360 degrees in 24 hours. This is
equivalent to 1 degree in 4 minutes or 15 degrees in one hour.
The standard time longitudes or meridians range from 45 to 165
degrees west in increments of 15 degrees. Therefore, the time at
each meridian is one hour earlier than the previous meridian.
To find your local time, first find the distance in degrees
between your longitude and your time meridian. Multiply the
number of degrees by four to obtain the correction for your
location. If you are east of the time meridian, add the
correction to the standard time for your area; if you are west of
the meridian, subtract the correction from the standard time. The
result is known as your local mean time.
For example, the longitude for Omaha, Nebraska, is 96
degrees, 6 degrees west of the standard time meridian of 90
degrees. This means the time correction is 6 x 4 or 24 minutes.
Since Omaha is west of the meridian, 24 minutes must be
subtracted from Central Standard Time to arrive at the local mean
time for Omaha.
Over the course of a year, the Earth's orbit causes the sun
to run either ahead of or behind local mean time by as much as 16
minutes. The actual difference between local mean time and the
actual or apparent time is called the equation of time. Computer
programs that compute the sun's angle, such as the TERC VHS-1
spreadsheet, automatically determine and correct for the equation
of time. For additional information, see Further Reading.