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May 08
Astronomy year numbering is mainly based on AD i.e. Anno Domini and CE i.e. Common Era year numbering. Here the decimal integer numbering is followed more strictly. There will be year 0 and the years before this are indicated by a minus sign. Those era designations like BC i.e. before Christ are dropped. Therefore the year 1 BC is numbered 0 and 2 BC is numbered -1. In general it can be said that n BC is indicated as (1-n). But here the numbers of AD years are not at all changed. The suffix AD is instead not used. This is replaced by either no sign or a positive sign. Thus we can represent n AD as +n.
Due to its greater use in astronomy the system is named so. Only a small number of disciplines in history work with time before year 1. These include dendrochronology, archeology and geology. It is a fact that the absolute values of historical and astronomical years differ by just 1. But this fact is severe when calculating events like eclipses and planetary conjunctions.
The zero year concepts was first introduced by French astronomer Philippe de La Hire and Jacques Cassini in 18th century. Along with their zero year versions they also used AD/BC designations. Thus the years were exposed as 2BC, 1BC, 0, 1AD, 2AD. They did not used -/0/+ in their versions. During the nineteenth century astronomers designated years with BC/0/AD or -/0/+. They did not fully use the concept of -/0/+ until the middle of twentieth century.
The completed calendar will consist of 4 lines. Three of the lines are the ones that are traced by a poles shadow on summer solstice which is on 21st of June, the equinox on March 21st and September 21st and the winter solstice on December 21st. the 4th line is one from the pole perpendicular to each of those lines drawn on the solstices and equinox. The apparent noon is marked by this i.e. the exact midpoint of the day. This noon line is always pointed towards the true north pole of the earth.
The astronomy calendar is a neat guide on what appears on the sky at night time. This includes planetary positions, lunar occultation, eclipses, conjunctions, meteor showers etc. there are many other features provided by the calendar. Here star maps show the planets in pre dawn and post sunset sky. This especially shows the conjunctions of the planet. Again a summary of events regarding the best suited time to view planets and other events are specified. There is a daily reminder of conjunctions, eclipses and meteor showers. We can also find forecasts of lunar occultation of stars and planets which indicates where and when they can be viewed.
Thus an astronomical calendar lists all those events which are predictable and cyclic. Event date and time are given in 24 hour clock format GMT. Depending on ones location the local time and date can differ from the GMT.
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