- Sailors have always been able to navigate using the Moon, despite the Moon constantly changing its position in the sky and waxing and waning until it is no longer visible.
- This because the Moon is always visible except for 4 nights in a month, it acts as a compass, as a clock, as a calendar and it provides light for 4 nights every month.
Introduction
- The Moon travels through the Ecliptic, along the same path as the Sun.
- Like the Sun and the Stars, the Moon rises in the East and sets in the West.
- However, the Moonrise is up to plus or minus 30 degrees from True East, and then Moonset is up to plus or minus 30 degrees from True West. This is due to the elliptical orbit of the Moon.
- A Full Moon always Rises in the East almost exactly opposite to the setting Sun in the West. On the Equinoxes, this is almost exactly Due East.
The Eight Phases of the Moon
- Eight Phases of the Moon
- New, Waxing Crescent, First Quarter, Waxing Giboous.
- Full Moon, Waning Gibbous, Last Quarter, Waning Crescent.
The Moon acts as a Compass
For Navigation at Night, the Moon will give an approximate guide to East, South and West. The Moon is only invisible for 3 to 4 days each month during the New Moon Phase.
- East
- Like the Sun, the Moon will always rise in the Eastern sky.
- West
- It will always set in the Western sky.
- South
- A line joining the points of the Crescent Moon to the Horizon will always point due South in the Northern Hemisphere.
- (And due North in the Southern Hemisphere).
- Varying position of Moonrise or Moonset
- However, the position of Moonrise and Moonset will vary from True East or True West by up to plus or minus 30 degrees.
- This is due to its elliptical orbit known as the Anomalistic Month, which takes 27.5 days to complete.
- Varying time of Moonrise or Moonset
- Unlike the Sun, the time of Moonrise and Moonset will vary according to the Moon’s cycle known as the Synodic Month of 29.5 days.
- And also, the time will vary according to the observer’s Latitude and Longitude.
The Moon acts as a Calendar
- The Time taken for the complete Moon cycle from the New Moon to the next New Moon is always 29.5 days.
- This is known as the Synodic Month.
- The time from New Moon to Full Moon is approximately two weeks.
The Moon acts as a Clock
The Tables in an Almanac are constructed for predicting the local times of Moonrise and Moonset throughout the year, as well as the times of rising and setting of the other planets.
- The New Moon
- Rises at almost the same time as the Sun.
- Sets at almost the same time as the Sun.
- The First Quarter Moon
- Rises at mid-morning
- Sets at Midnight.
- It is at its highest around Sunset.
- The Full Moon
- Rises at Sunset
- Sets at Sunrise.
- It is at its highest around midnight.
- The last Quarter Moon
- Rises around Midnight
- Sets around mid-morning.
- It is at its highest around Sunrise.
The Moon provides light
- The Full Moon
- Reflects the Sun’s light during the whole night.
- The intensity of the Moonlight varies according to its altitude in the sky, its distance from the Earth (it has an elliptical orbit) and the prevailing atmospheric conditions.
- Brightness (4 days)
- Moonlight reaches its maximum brightness on the night of the Full Moon.
- It then falls to half that brightness during the two days on either side of the Full Moon.
- Therefore, the reflected Moonlight is useful only during a four day period during the Moon’s cycle.