THE PLANET MARS AND THE DISTANCE OF THE SUN FROM THE EARTH.

New Zealand Times, Volume XXXII, Issue 5132, 4 September 1877, Page 3

THE PLANET MARS AND THE DISTANCE OF THE SUN FROM THE EARTH.

(From the “Wellington Almanack, 1877.”) This year will be a marked one for astronomers, as it is expected that by careful measurements of the planet Mars from certain fixed stars, the Earth’s distance from the Sun will be accurately gained. It is believed that the observations taken with such extreme care bn the transit of Venus, in December, 1874, are not altogether trustworthy. The great difficulty is to get the exact time when Venus, on entrance, leaves the Sun’s disc, and when on exit, she touches it. The motion of the planet is very slow ; and further, when there is but a small distance between the edges of Venus and the Sun, the intervening space appears to be-, come clouded. Photographs were taken which were free from these disturbances, but it is now known that the size of the disc of the Sun taken by photography is larger than the measured size of the disc. Another source of error was to be found in the nervous anxiety of the observers, as the phenomenon to be observed was of such deep importance, and of such uufrequent occurrence. Observations on Mars will be free from these disturbing influences. Any number of measurements of the distance of Mars from a star can taken by astronomers at opposite sides of the world, and.any pair of these will give the desired result ; so that there is not one chance only to be secured, but very many to be leisurely noted. Again, the edge of the disc Mars is very sharp, and its brightness will not fatigue the eye. This sort of work is done repeatedly in every observatory, in micrometer measurings of the distance separating one star from another. Thus the wor-lc is simple, can be done leisurely, and the result will be especially trustworthy. From the old transits of Venus the Bun’s distance was placed at 95,000,000 miles; but within the last few years, and from other plans of gaining this distance, it has been discovered that this distanse was too great. The mean distance is about 92,000,000 miles. The method to be followed is a vei-y simpleone. Kepler proved that all the planets arc so intimately connected together^that, for any two, the time of the revolution around the Sun, or the period of one planet, is to the period of another as the cube of the distance from the Sun of one is to the cube of another’s distance. The Earth’s period is 365 days, and the period of Mars 687 days. This proportion will give for Mars’-distance from the Sun, if the Earth’s distance bo called 1, 1.52,; and thus, if the distance of the Sun from the Earth be divided into 100 spaces, Mars will be placed at the 152nd. When observations of Mars are taken from a star by observers at stations in each hemisphere, as far distant as possible, the planet will not appear to be at same distance from the star to each observer, or’ Mars will have, iu astronomical phrase, a large parallax. Mars in 1877 will, be almost ,as near to the Earth as it can be, so' that these observations will be exceptionally valuable, as the, nearer the object observed is to, the observer the larger the parallax. From this parallax so'gained the distance of Mars from the Earth can be readily calculated ; and now, the, value of the 52 spaces from the Earth being known, the value of the 100 spaces from the Barth to the Sun are also known.

Mara will be brightest on September SJ : and will then shine as a very, large red. star, easily distinguishable from its color and absence of twinkling. It will be first visible about April, in the evening, and will gradually increase in size and)brilliancy. Any good telescope; will show readily the seas and continents of the planet. It will be pleasant to watch the increase in the planet’s brightness and its rapid motion among the fixed stars.