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  2. Lunar phase - Wikipedia

    en.wikipedia.org/wiki/Lunar_phase

    [8] [d] The difference between two dates can be calculated by subtracting the Julian day number of one from that of the other, or there are simpler formulae giving (for instance) the number of days since 31 December 1899.

  3. Equinox - Wikipedia

    en.wikipedia.org/wiki/Equinox

    Equinox. A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.

  4. Calendar year - Wikipedia

    en.wikipedia.org/wiki/Calendar_year

    The Lunar Hijri calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars. A year can also be measured by starting on any other named ...

  5. Gregorian calendar - Wikipedia

    en.wikipedia.org/wiki/Gregorian_calendar

    The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar.

  6. Solstice - Wikipedia

    en.wikipedia.org/wiki/Solstice

    17:37. 21. 14:14. A solstice is the time when the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20-22 June and 20-22 December. In many countries, the seasons of the year are defined by reference to the solstices and the equinoxes.

  7. Calendrical calculation - Wikipedia

    en.wikipedia.org/wiki/Calendrical_calculation

    The number of days between two dates, which is simply the difference in their Julian day numbers. The dates of moveable holidays, like Christian Easter (the calculation is known as Computus) followed up by Ascension Thursday and Pentecost or Advent Sundays, or the Jewish Passover, for a given year. Converting a date between different calendars.

  8. List of environmental dates - Wikipedia

    en.wikipedia.org/wiki/List_of_environmental_dates

    09: Third Friday in September. World Cleanup Day [135] 09: September 16, 2023, Third Saturday in September. National Cleanup Day [136] 09: September 18, 2021, Third Saturday in September. International Red Panda Day [137] 09: Third Saturday of September. Zero Emissions Day [138] 09-21: September 21.

  9. Julian day - Wikipedia

    en.wikipedia.org/wiki/Julian_day

    The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date). [1]