Hands On Physics


Concepts overview

The Sun's Angle

Every Sun photometer light intensity measurement must be accompanied by a measurement of the Sun's angle above the horizon.

It is important to know the Sun's angle because the distance sunlight has to travel depends on this 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 = 1 ). When the Sun is closer to the horizon, its rays pass through more atmosphere than when the Sun is straight overhead. Thus the airmass m increases as the Sun moves closer to the horizon. The air mass mfor each of your measurements is 1 divided by the sine of the Sun's angle.

Analyzing Your Calibration Data

You can use the data from your Langley test to find what's called the extraterrestrial constant, the signal the TERC VHS-1 would give at the top of the atmosphere (4). In other words, if you took the TERC VHS-1 along on a trip into Earth orbit in the Space Shuttle, the signal you would measure using the TERC VHS-1 during a space walk should be very close to what you measure on the ground using the Langley method.

Some Sample Photometer Data

Measurements taken with a Sun Photometer since 1960 show the results of Mount Pinatubo's erruption.

The Haze Formula

Every TERC VHS-1 has its own distinctive ET constant. This calibration number permits you to determine what scientists call the Aerosol Optical Thickness (AOT) of the atmosphere. AOT provides an excellent haze index. Even though the ET constant of different instruments may vary, all instruments should give very similar AOTs.
The AOT formula is incorporated into the TERC VHS-1 spreadsheet which can be downloaded from: ftp://concord.org/pub/haze/vhs1-download/


The modern era of Sun photometry began with the development of hand-held instruments by Frederick E. Volz, and continues with your work. Some important books and journal articles about haze are referenced here.

Beer's Law

Beer's Law is a mathematical model which can be used to describe light transmission through increasing thickness of air.


Definitions and explainations of some of the terms used in this unit


The basic design of the TERC VHS-1, portions of this HOP unit, and the accompanying software are adapted from the copyrighted publications by Forrest M. Mims III listed as references 3-5. All such materials are used with the permission of the author.

Development of the TERC VHS-1 was supported by a grant from the National Science Foundation. The project was initiated by Barbara Tinker, Boris Berenfeld and Robert Tinker of TERC and the Concord Consortium.

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