Solar Time

SOLAR TIME Solar Time, or sun time, is determined in two ways. Apparent Solar Time is the system of days and hours which goes strictly by the sun itself and is thus continually changing in value from day to day. It is the time between two successive transits of the sun over the same meridian. Mean Solar Time is the system of days and hours mathematically computed in order to give the average value to every hour and day. It is 24 hours. The difference in value between apparent and mean solar time is known as equation of time.

SIDEREAL TIME OR STAR TIME Whereas the sun moves sometimes slow and sometimes fast, with a total range of half-an-hour from one extreme to the other, the stars provide a perfect time-piece. But they do not operate according to the conventional systems of days and hours that our calendars follow. A star takes 23 hours 56 minutes of mean solar time and 4.09 seconds to complete one rotation of the earth, covering 360°. This interval is called a sidereal day, which is thus about 4 minutes shorter than
the mean solar day of 24 hours.

THE MOON: EARTH’S ONLY SATELLITE

Salient features of the moon, the earth’s only satellite,
are as follows:
. The moon is earth’s only satellite.
. The mean distance between the earth and the moon
is about 3,85,000 km.
. Moon has a diameter of about 3,480 ‘km and a mass
1 of about 81 that of the earth.
. The orbit of the moon is elliptical.
. The time taken by the moon to complete one revolution around the earth is 27 days, 7 hours, 43 minutes and 11 1/2 seconds, or about 273 days. (This
period is called sidereal month.)

.The period of moon’s revolution of the sun is 29.53 days on an average, and is called synodic month.
. The moon’s period of rotation around its axis and revolution round the earth is same.
. Moon at all times keeps the same side towards the
earth.
. The plane of the moon’s orbit is inclined at an angle
of 5° 09’ to the plane of the ecliptic.
. When the sun and the moon lie on the same side
of the earth, the moon is said to be in conjunction
with the sun.
. When the sun anc:l. the moon are on opposite sides
of the earth, they are sail.:! to be in opposition.
. The major cause of sea-tides is the gravitational pull of the moon. The sun, because of its greater distance from the earth, has a tide-producing power that is only five-elevenths the tide-producing power of the moon.
. When the moon is between the earth and the sun, the position is called the New Moon. On New Moon, the part of the moon facing the earth is in complete darkness. The moon takes different shapes on different days after the New Moon: waxing crescent (after 3 days), first quarter (7th day), waxing gibbuns (10th day), full moon (14th day), and waning gib bous (17th day), last quarter (21st day), and waning crescent (25th day).

ECLIPSES

The total or partial obscuration of light from a celestial body as it passes through the shadow of another body is known as eclipse. On earth we are familiar with the solar and lunar eclipses. The moon, as a satellite of the earth, revolves round it; in the process it is bound to come in- between the sun and the earth at times. When both sun and moon are on the same side of the earth (i.e., in conjunction) so that all three bodies lie approximately on a straight line, the possibility exists for an eclipse of the sun, or solar eclipse.

It is a rare occurrence because the moon is so small and the plane of its orbit is tilted about 5° with respect to the plane of the ecliptic. It is for this reason that eclipses do not occur every month.’ (Ectiptic is the apparent track of the sun throughout the year as a result of the earth’s motion around it. The plane of the ecliptic is the plane passing through this path coincident with the place of the earth’s orbit, and is imagined to be horizontal, passing through the globe’s centre.) }Vhen the moon and sun are on the opposite sides of the earth (Le., in opposition), the possibility exists for an eclipse of the moon, or lunar eclipse. In this case the earth’s shadow falls on the moon partly or completely covering it for a short while.

A solar eclipse occurs between sunrise and sunset at new moon; a lunar eclipse occurs at full moon. The chances of our seeing a lunar eclipse from a given place on earth are much better than for seeing a solar eclipse. In one year, up to se:ven eclipses can occur; either five solar and two lunar, or four solar and three lunar.
Incidentally, in a solar eclipse, the shadow of the moon
is some 45 km wide and crosses a belt of the earth several kilometres long.

In a lunar eclipse the earth’s shadow is far larger than the moon and can cover the moon’s surface for about 3 hours. A total lunar eclipse may last up to 1 hour 40 minutes. The moon does not become completely dark during most lunar eclipses. In many cases, it becomes reddish. The earth’s atmosphere bends part of the sun’s light around the earth and towards the moon.’ This light is red because the atmosphere scatters the other colours present in sunlight in greater amounts than it does red, In the case of a total solar eclipse, the totally darkened period may be as long as 7 minutes and 40 seconds, but the average is about 2Vz minutes. The path of totality is wide but not wider than about 274 kilometres.