Time can be thought of as a system of measuring duration, e.g., years, months, days, hours, minutes, seconds. Astronomical time refers to the mean solar time reckoned by counting the hours continuously up to 24 from one noon to the next or time by the mean sun, from local mean midnight through 24 hours to the next midnight. The mean sun refers to a point moving eastward along the celestial equator at a rate equal to the arithmetic average rate of the sun’s motion along the elliptic.

Apparent solar time is marking time by the apparent sun (the sun as seen by an observer on earth). The reckoning of apparent time, is from local apparent midnight (12 pm) through 24 hours to the next midnight. The apparent sun does not provide a constant solar day. The apparent solar day varies in length since the earth does not revolve around the sun at a uniform rate and the inclination of the ecliptic is toward the celestial equator.

Archaeologists, astronomers, and Egyptologists often use Julian dates as a standardized form of timekeeping. Accordingly, we use the Rosetta Calendar accurate, accessible, and useable for the conversion of Gregorian, Julian, and Hebrew calendar dates.

For purposes of biblical study and Bible lands archaeology three types of calendars bear consideration  – solar, lunar, and luni-solar. A solar calendar, such as the Julian calendar and the present day Gregorian calendar, has as its basis the solar year. The early ancient calendars, such as that of the ancient Egyptians, consisted of lunar calendars of twelve months with each month of thirty days. The luni-solar calendar, as that of the Hebrews, replaced the lunar calendar bringing the solar year and the lunar month into a closer relationship. Ancient calendars become helpful in archaeology when linkages occur between the ancient calendars and the Julian calendar.

All calendars have an astronomical basis. The natural progression of the earth around the sun and the moon about the earth provide a convenient means of marking time. Ancient peoples often used observed cyclical natural phenomena, based on astronomical laws such as the annual flooding of the Nile, to establish their calendars. The Romans developed their calendar on the period of time spent by the sun on its apparent passage from vernal equinox to vernal equinox. The Hebrews marked time based upon the movements of the sun and the moon. Today astronomers chose to mark apparent time by using the vernal equinox as the time reckoner.

The solar system consists of the sun and all the heavenly bodies that revolve around it such as planets and comets. A single revolution of the earth about the sun marks the passage of one solar year. In the ordinary use of English a “year” refers to the “apparent solar year” but in a scientific sense a year may be a solar year, a lunar year, or a sidereal year.

Years

A solar year (365.2422 days) consists of the period of time (365 days, 5 hours, 48 minutes, and 46 seconds) spent by the sun in making its apparent passage from vernal equinox to vernal equinox. In fact, a solar year is the time it takes the earth to make one orbit about the sun using the instant in the spring when the sun’s center crosses the equator as a reference point.

The earth, however, does not circle the sun at a constant rate, nor does the earth rotate on its axis at a fixed rate. The earth revolves faster when it is nearer the sun during the northern winter. The solar year, or the year of the seasons, is about 20 minutes shorter than the true period of the earth’s revolution. The difference is the result of the precession of the earth. An anomalistic year is the time a planet takes to go from perihelion (the point in the orbit of the earth nearest the sun) to perihelion. A solar year is also known as an astronomical, equinoctial, natural, or tropical year.

The vernal equinox refers to the time in the spring (about March 21), and the autumnal equinox to the time in the autumn (about September 22), when the sun crosses the celestial equator making night and day of equal length all over the earth. The solstice is the time at which the sun reaches either of the two points on the sun’s ecliptic at which it is farthest north (summer solstice about June 21) or furthest south of the equator (winter solstice about December 22). These phenomena mark the change of the seasons–spring (vernal equinox), summer (summer solstice), fall (autumnal equinox), winter (winter solstice). The apparent solar day with the period of most daylight in the northern hemisphere is at the summer solstice. The day with the least is at the winter solstice.

A sidereal year (365.2563 days) consists of the period of time (365 days, 6 hours, 9 minutes, and 10 seconds) of mean solar time spent by the sun in its apparent passage from a fixed star and back to the same position again. The difference in time between the sidereal year and the solar year of the equinoxes is due to the precession of the earth. Measurement of astronomical time is in sidereal, from the Latin sidereus “a star,” time. The astronomical day is from noon to noon.

Precession is the result of the action of the gravity of the sun and moon upon protuberances about the earth’s equator. The precession is the occurrence of the equinox earlier in each successive sidereal year, brought about by a gradual westward movement of the equinoctial points along the ecliptic as the result of the change in direction of the earth’s axis as it turns around the axis of the ecliptic so as to describe a complete cone approximately every 26,000 years.

A lunar year (365.2600 days) consists of 12 lunar months. In a period of about 19 years (almost 235 lunar revolutions) the new moon reappears on the same day as it did at the beginning of the cycle. In 19 solar years there are 228 solar months (12 x19) which is 7 months short of the 235 lunar months in 19 lunar years. Reconciliation of the seasons with the calendar requires adding 7 extra months to adjust the solar calendar to the lunar calendar [(12 lunar months per year x 19 years) + 7 lunar months = 235 lunar months]. Intercalary refers to a year, month, or day added to a calendar, as in a leap year, to make a calendar correspond to the solar year. Interpolated or inserted means to enlarge by putting in new material such as a day.

The mathematically based Hebrew Perpetual Calendar, due to the small drift of about a day every 231 years compared to the Gregorian calendar, is about 15 days too long or late relative to the equinox relationship that it originally had at the Exodus (1446 BCE). So on the Hebrew Calculated Calendar the Feast of Trumpets falls on the average about two weeks later than it did in 1446 BCE.

Months

A “month” normally refers to a “synodic month” consisting of the period of time from one new moon to the next equivalent to 29 days, 12 hours, 44 minutes, and 2.7+ seconds. The moon appears completely eclipsed as seen from earth. This eclipse became known as a new moon because to the ancients it appeared that the moon experienced a new birth after every eclipse.

The time for the new moon came to be known in Hebrew as the molad, literally meaning “renewal.” Instead of minutes and seconds, in the Hebrew calendar, the hour was divided into 1080 parts (halakim) with each part corresponding to 3 1/3 seconds. The traditional time given for one lunation, the period from one molad to the next, is 29 days, 12 hours, and 793 parts. The Hebrew year normally consists of twelve of these months.

In the Hebrew calendar the months closely followed the moon from molad to molad but it also followed the solar year. This luni-solar calendar depended on the cycles of the moon, 29 and 1/2 days long, and the solar year, 365 and 1/4 days long. Rather than adding a day, as in done in the Gregorian calendar, the Hebrew calendar occasionally added a month. The calendar requires other corrections as well to keep the seasons in line with the calendar. These corrections and postponements, developed to minimize weekly and annual Sabbaths from falling on contiguous days, and whether the priests utilized calculations or astronomical observation to determine the time of the molad, and whether this was in the spring or fall remain unresolved issues.

The sacred year began in the spring with the month called Nisan. The civil year began in the fall with the month of Tishri. In our present day Tishri 1 is known as Rosh Hashanah. The first of Tishri does not always occur on the day of the molad because of dehioth (postponements).

A “lunar month” refers to the period of a complete revolution of the moon with reference to some fixed point. An anomalistic month is the time that the moon takes to go from perigee (the point nearest to the earth in the orbit of the moon) to perigee. The “new moon” is the precise instant when the moon in conjunction with the sun becomes visible as a narrow crescent.

A conjunction is the condition or state of being in the same celestial longitude. This places the three spheres (earth, moon, and sun) in a single plane but not necessarily in direct alignment. The “full moon” refers to the time of the illumination of the entire disk of the moon visible as a circle. The “old moon” or “waning moon” refers to the moon any time after it has ben full. The mean time between a new moon and the next is 29.530588 days.

Days

A “day” consists of the period of time, defined as exactly 24 hours, necessary for the earth to complete a single rotation on its axis. More specifically it is the interval between two successive transits of the time reckoner over the same branch of the celestial meridian. A sidereal day (23 hours 56 minutes 4.09 seconds of mean solar time) is the interval between two successive transits of a star over the meridian.