Significance of Solar Eclipse
Why do astronomers travel the world to see eclipses? Firstly, because the Sun may be likened to a physics laboratory in space. In its vicinity, conditions of temperature ,pressure, gas density, and so on, prevail which are not reproducible on Earth and, during the eclipse, the Sun’s extended envelope, the corona, is accessible for observation and experimentation. Secondly, the study of the Sun is a natural and integral part of our study of the Universe: if we would understand the physics of the billions of stars scattered through space, we must first of all understand the Sun and its environment ‘on our doorstep’. Finally, as we have already seen, Sun-Earth interactions directly affect all of us.
But what aspects of solar eclipses interest scientists and what are their motivations for observing them?
Observation of the solar corona
On clear days the sky is too bright around the solar disk – even from mountainous regions – to allow observation of the whole corona in white light; but during a total eclipse of the Sun, sky brightness is reduced by a factor of between 1,000 and 10,000. For observers in the Moon’s shadow, sky brightness is due only to the scattering of light reflected by the ground and by the lower atmosphere in the area surrounding the shadow. This brightness will vary as the Moon’s shadow passes rapidly across. During the eclipse, the sky will be dark enough for structures of the solar corona to be seen and studied.
Other possibilities for study
Other measurements, of less import and interest than those of the corona, can be taken during eclipses of the Sun.
Measuring the Sun’s diameter
Measuring the duration of the eclipse allows us to measure the Sun’s diameter. Account must be taken of the profile of the lunar limb and the observer’s position vis-à-vis the centre line, as corrections involving the relative positions of the lunar and solar disks will be needed if the observer is not on the centre line. Accuracy in the measurement of this interval must be to within 0.01s, corresponding to 0.005 arcsec of the solar diameter, or 4 km in 1,391,000. Such measurements are used to verify hypothetical variations in the diameter of our star.
Measuring the shape of the Moon
The Moon is an ellipsoid with three axes. The major axis points towards the Earth, while the other two are at right angles to this axis. Eclipses of the Sun have resulted in very precise measurements of the shape of the Moon, by observers stationed on either side of the centre line.
The progress of a total eclipse is marked by contacts. First contact is the moment when the Moon takes the first ‘bite’ out of the Sun’s disc. Second contact is the disappearance of the last gleam of the Sun’s light: a great moment for the observer, at the onset of totality. When totality ends, seconds or minutes later, the Sun reappears in a burst of light as third contact occurs. Fourth contact is the moment when the Moon moves completely off the Sun, which resumes its circular aspect.
The solar disk is considered to be circular, and it is possible to determine certain lunar dimensions by measuring the time elapsed between second and third contacts, thus increasing our knowledge of the Moon’s geometry.