PLANETS IN AUGUST

Press, Volume XCIX, Issue 29270, 30 July 1960, Page 10

PLANETS IN AUGUST

[By C. S. L. KEAY, University of Canterbury]' The brilliant planet Venus will this month appear in the early evening sky, where it will remain until the beginning of 1961. Since it is by far the brightest object in the night sky, with the exception of the moon, it cannot fail to attract attention, although it will be rather low in the western sky for the first part of the month. For most of August, Venus will remain in the constellation Leo, passing near the bright star Regulus on August 9, and into the constellation Virgo on August 28. Venus sets at 6.17, 6.54, and 7.33 p.m. on August 1, 16, and 31 respectively. Next in order of brilliance is the giant planet, Jupiter, which in the evening is high and prominent in the Sky. Not far from it in an easterly direction is the ringed planet Saturn. The rings of Saturn and the ever-changing configurations of the four Galilean satellites of Jupiter provide endless satisfaction to possessors of small ’ telescopes. The red planet. Mars, remains in Taurus, not far from the Pleiades, the “Seven Sisters,” as they are often called. On the days of the month mentioned above Mars will rise at 2.57, 2.47, and 2.35 a.m.; Jupiter will set at 5.02, 4.00, and 3.02 a.m„ and Saturn will set at 6.20, 5.18, and 4.17 a.m. respectively. In the cases of Jupiter and Saturn it is fairly true to say that they set four minutes earlier each evening. Using this rough rule the setting times on the dates not given may be estimated. Tides Now that spring is not far away the subject of spring tides comes to mind. Indeed tides in general puzzle most people. Everyone can appreciate that the moon’s gravitational attraction causes the water of the oceans to rise on the side of the earth underneath the Moon, but it is harder' to 'understand why a high tide should also occur on the opposite side of the earth. The explanation lies in the fact that -the earth is a very rigid globe and moves as a whole under the attractive force of the moon. In other words all parts of the earth move as though the force acting was everywhere equal to that at the centre of the earth. The oceans, not being rigid like the earth, experience a .force which, directly underneath the moon, is greater than that at the centre of the earth, and, on the opposite side away from the moon the oceans experience a lesser force than acts at the centre of the earth. The lesser force allows the tide on the side of the earth away from the moon to rise by the same amount as the tide directly under the moon rises by the greater force -acting on the moonward side. A more detailed discussion would show how the moon’s attraction gives rise to the low tides as well. The so-called “spring tides” occur whenever the sun and moon are in a straight line with the earth, he. at hew moon and full moon. Thus they are a fortnightly phenomenon. The sun has rpughly half as much tide-raising effect as the moon, and when the two act together they can raise a tide about one and a half times normal. However, the timing and height of tides are influenced by so many other factors, such as the shape of ..the coastline, the depth of the.sea nearby and the elevation of the sun in the sky, that the detailed study of tides is a difficult and involved task.