Julian dates

Julian dates (abbreviated JD) are simply a continuous count of days and fractions since noon Universal Time on January 1, 4713 BCE (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are widely used as time variables within astronomical software. Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision. Note that the time scale that is the basis for Julian dates is Universal Time, and that 0h UT corresponds to a Julian date fraction of 0.5.

It is assumed that 7-day weeks have formed an uninterrupted sequence since ancient times. Thus, the day of the week can be obtained from the remainder of the division of the Julian date by 7.

Calendar dates — year, month, and day — are more problematic. Various calendar systems have been in use at different times and places around the world.The Gregorian calendar, now used universally for civil purposes, and the Julian calendar, its predecessor in the western world. As used here, the two calendars have identical month names and number of days in each month, and differ only in the rule for leap years. The Julian calendar has a leap year every fourth year, while the Gregorian calendar has a leap year every fourth year except century years not exactly divisible by 400.

The changeover from the Julian calendar to the Gregorian calendar occurred in October of 1582, according to the scheme instituted by Pope Gregory XIII. Specifically, for dates on or before 4 October 1582, the Julian calendar is used; for dates on or after 15 October 1582, the Gregorian calendar is used. Thus, there is a ten-day gap in calendar dates, but no discontinuity in Julian dates or days of the week: 4 October 1582 (Julian) is a Thursday, which begins at JD 2299159.5; and 15 October 1582 (Gregorian) is a Friday, which begins at JD 2299160.5. The omission of ten days of calendar dates was necessitated by the astronomical error built up by the Julian calendar over its many centuries of use, due to its too-frequent leap years.

The changeover to the Gregorian calendar system occurred as described above only in Roman Catholic countries, however. Adoption of the Gregorian calendar in the rest of the world progressed slowly. For example, for England and its colonies, the change did not occur until September 1752. (The Unix cal command for systems manufactured in the U.S. reflects the 1752 changeover.)

Why use Amateur Radio in Emergency Communication?

why Amateur Radio is so important during emergencies and disasters, The following should make you understand

Since Amateur Radio equipment does not rely on wires and communications facilities provided by common carriers and phone companies, it is immune to disruptions with the telephone system. Licensed Radio Amateurs use a wide range of radio bands, each one with its particular strength in overcoming the barriers to radio communications. VHF (Very High Frequency -- 144 MHz) and UHF (Ultra High Frequency -- 440 MHz) radios are small and portable with lots of channels to handle a multitude of short-range communications.

HF (High Frequency) can propagate over mountains and valleys and between islands to provide coverage beyond VHF and UHF. Hams can use a wide range of communications modes, whether TV, data, voice or Morse code to exchange messages. The very nature of the Amateur Radio service encourages amateurs to learn how to make contacts, regardless of the challenges that may abound.

Amateur Radio operators are distributed throughout the community and the nation, near schools,offices, buildings and park /open grounds facilities which are often used for evacuation shelters. Many companies have Amateur Radio operators within their employment. They may already be near the scene of the trouble area and can respond. Should for any reason, a section of the town or city be devastated, there are enough equipment and operators ready in other parts of the community to respond.

Regardless of the specific brand and model of radio equipment, amateurs using the same frequency band and mode can communicate with each other. Amateur Radio has more flexibility in the frequencies, range, modes, and kinds of persons with whom one many communicate -- which is the experiential advantage of Amateur Radio.

Hams are already licensed and pre-authorized to communicate internationally into and out of places hard hit during natural disasters.

Amateur Radio operators are allowed to run higher power than other licensed and unlicensed personal radio services such as Citizen Band (CB) and the Family Radio Service (FRS) and have more flexibility with the equipment. Therefore, hams can communicate over greater distances.

Amateur Radio operators use their equipment regularly, which verifies that it’s maintained and operational. Some of the equipment includes hand-held portables or mobiles installed in vehicles. The Amateur Radio operators are familiar with the operation and capabilities of their equipment, and how to overcome obstacles to radio communications that may exist within their neighborhood.