VELOCITY OF LIGHT.

Hawera Star, Volume XLVIII, 13 September 1924, Page 5

VELOCITY OF LIGHT.

how it was ascertained; Light moves with,the- -amazing velocity of 185,000 miles a second,- a speed a ; million times as great as ’ that r of a rifle bullet. It would make the oilcuit of the earth’s circumference', • at' : the equator j seven... times-in-one beat. :,of the pendulum. For a long- time light was thought to be instantaneous, but it is now known to have a'measurable velocity. The discovery was first made by means of eclipses of Jupiter’s . satellites. Jupiter, like the earth, casts a shadow, and when his moons ' pass through it they are eclipsed; just : as our moon, is .eclipsed when- passingthrough the earth’s shadow. Jupiter’s' shadow far surpasses in magnitude that of the earth. His moons revolve‘around him much more rapidly than our moon revolves around the earth,- and- their orbits are nearly in the plane of the planet’s orbit. Consequently they all, with the exception of the fourth' aild most distant satellite, pass through the' planets ghadow and . are eclipsetL iat every revolution. Roemer, a. Danish astronomer, made in 1675 some curious observations in regard to the-times: of the occurrence of these eclipses. - Whe u Jupiter is nearest the earth, the eclipses occur about sixteen minutes earlier than when he is most distant from the earth. The difference in distance between the two points is about 185,000,000 miles, the diameter of the earth’s orbit, or twice her distance from the sun. It takes light, therefore, sixteen minutes to traverse the diameter of the earth’s orbit, and half tnat time to span the distance between, the sun and the earth. Light is thus shown to travel 185,000 miles a sec- : ond, and to take eight minutes—or more exactly, 500 seconds—in coming from the sun to the earth. . It follows that we do not see the sun until eight minutes after sunrise, and that we do ff® llim eight minutes after sunset., \\hen we look at a star we do. not see the star as it now is, but the star as . it was several • years ago. It takes light three years to come to us from the nearest star, and were it suddenly blotted from the sky, we should see it ' shining there for three years to come. Ihere are other methods of finding the velocity of light, but the satellites of -J upiter first revealed its progressive movement. -• • •