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that matter is being continuously created, at the rate of a few hundred atoms per year. This was supposed to allow the density of the Universe to remain constant as it expands.

13.01 Einstein, the developer of the General Theory of Relativity, believed that the Universe was ageless and eternal.

13.02 A beginning would demand explanation of a cause. If there were no previous event, there could be no explanation of the cause. Therefore, the Universe is ageless and eternal. Time always existed.

14.01 Common general terms for periods of time are: Past, Present and Future. Other terminology would be Eternity Past, Recorded Time, Eternity Future.

15.01 A relationship may be so distant as to not be evident. You have a relationship to grandparents that would be hundreds of generations in the past - even though you may not have a direct knowledge of this relationship.

16.01 All periods of time are both real and relative. Definitions and divisions of the various periods of time are given in following paragraphs to promote understanding that time is both real and relative.

17.01 Day: (1a) the period of light between sunrise and sunset (b) daylight (c) sunshine.

17.02 (2a) the 24-hour period (mean solar day) that it takes the earth to rotate once on its axis with respect to the sun: the civil or legal day is from midnight to midnight, the astronomical day from noon to noon.

18.01 Sidereal day: the time between two successive passages of the vernal equinox across the meridian: it measures one rotation of the earth and equals 23 hours, 56 minutes, 4.1 seconds of mean solar time.

19.01 Sidereal month: the time required for the moon to complete one revolution around the earth with respect to a fixed star: its average value is 27.32 days of mean solar time.

20.01 Year (1) n. a period of 365 days (in leap year, 366 days) divided into 12 months and regarded in the Gregorian calendar as beginning Jan. 1 and ending the following Dec. 31 b) a period of more or less the same length in other calendars

21.01 Solar year: the period (365 days, 5 hours, 48 minutes, and 46 seconds of mean solar time) spent by the sun in making its apparent passage from vernal equinox to vernal equinox: the year of the seasons: also tropical year or equinoctial year.

22.01 Sidereal year: the period (365 days, 6 hours, 9 minutes, and 9.54 seconds of mean solar time) spent by the sun in its apparent passage from a fixed star and back to the same position again: it is the true period of the earth's revolution, and the difference in time between this and the tropical year is due to the precession of the equinoxes.

23.01 Lunar year: a period of 12 lunar months, as in the Jewish calendar.

24.01 Anomalistic year: the period of time occupied by any planet in making one complete revolution from perihelion to perihelion: for the Earth, this period is 365 days, 6 hours, 13 minutes, and 53 seconds: it is slightly longer than the sidereal year due to the extra time needed to reach an advancing perihelion, the lag being caused by the gravitational pull of the other planets.

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