Astronomy and Calendars
Throughout history, people have used different types of calendars to help them know when to plant crops, choose the best hunting times, plan meetings and observe religious holidays. All calendars work by making it possible for you to organize time units by observing astronomical cycles. Months are based on the moon's orbit around the Earth, years are based Earth's orbit around the sun and days depend on how fast the Earth revolves around its axis.
Solar Calendars and the Sun
Solar calendars, such as the Gregorian calendar, track time using tropical years. A tropical year, also called a solar year, is the length of time between two vernal equinoxes. That time period is 365 days, five hours, 48 minutes and 46 seconds. Many people refer to a vernal equinox is the first day of spring. There is no U.S. law that forces people to observe Gregorian solar calendar dates. The use of that calendar dates back to 1751 when the United Kingdom told its colonies to use the Gregorian calendar.
Lunar Phases and New Moons
A new moon is the opposite of a full moon. As the moon orbits the Earth, its position relative to the Earth and sun changes, and the moon appears to go through phases. When the Earth sits between the moon and sun, people on Earth see a full moon at night. A new moon occurs when the moon sits between the sun and the Earth. New moons occur during the day, so you can't see them because of the sun's brightness. A quarter moon, on the other hand, occurs when the moon completes 25 percent of its orbit around this planet.
Because the moon circles the Earth in the same time it takes to rotate once, the moon always shows one face to the Earth. That's why you never see its far side. A new moon occurs every 29.5 days. Astronomers call the time between new moons a synodic month. All lunar calendars that people create base their months on the synodic month rather than the months you find on a solar calendar.