The points below need to be considered when making a lunar calendar:

  1. The moon orbits the earth in 29.5 daysmoonfig1
  2. The earth (with the moon) orbits the sun in 365.24 days
  3. One orbit of the moon round the earth is called a lunation (month)
  4. The lunar year is made of 12 lunations/lunar months (354 days)
  5. The moon is fully visible (full moon) when it’s on the opposite side of the sun and earth
  6. The moon is totally invisible (new moon) when it’s in between the sun and earth
  7. The moon has 27 days of visible phases and 2.5 days of invisible phases
  8. The moon travels round the earth in an elliptical orbit, so its sometimes closer or further from earth
  9. The moon becomes visible after 29 days in some months and after 30 days in other months
  10. The lunar month is never less than 29 days and never more than 30 days (it’s 29 or 30 days)
  11. The moon may become visible after 29 days in one country/region and after 30 days in another
  12. The 29 and 30 days months do not alternate, they appear in different combination in each year
  13. The lunar year usually has 6 months that are 29-days and 6 months that are 30-days
  14. The Islamic lunar (hijri) calendar is based on sighting the moon by the human eye on the 29th day
  15. If the moon is sighted on 29th day, the new month starts from next day, otherwise after 30 days

The modern visibility calculations can predict moon sighting fairly accurately but actual sighting (confirmation) is used for religious purposes, such as for Ramadan and Eid festivals.

Related Articles: Science of Moon Sighting, The Lunar Date of Full Moon & Hidden Phase of the Moon