Phases of the Moon
Everyone is familiar with the phases of the Moon, which are explained in the quaint ink sketch scanned from an 1873 edition of 'Astronomy' by John Isaac Plummer of the University of Durham. Although the Moon orbits the Earth in 27.32 days (sideral month), the time between each new Moon is 29.53 days (synodic month), with the Moon sweeping through 389° in the sky. In this time the Earth completes just under 1/12th of its own orbit around the Sun.
The Moon phase calculator is a useful tool for planning observing and imaging sessions. Generally the best time to photograph a lunar feature is within 2 days of lunar sunrise or sunset at the point of interest.
Best Observing Dates
Each lunar phase has a best and worst time for observation throughout the year. This is because the Moon roughly follows the ecliptic, the annual path that the Sun follows through the sky, so there are times when the Moon has a high (favourable) altitude or a low (unfavourable) altitude in the sky. These times are reversed for the Northern and Southern hemipheres, as are the seasons.
|Lunar Phase:||3-4 days||First Quarter||Full Moon||Last Quarter||25-26 days||Most Favourable:||late October||late September||late June||late March||late January|
|Least Favourable:||late April||late March||late December||late September||late July|
|Lunar Phase:||3-4 days||First Quarter||Full Moon||Last Quarter||25-26 days||Most Favourable:||late April||late March||late December||late September||late July|
|Least Favourable:||late October||late September||late June||late March||late January|
As shown below, the Moon's orbit is relatively complex. Four periodic cycles determine the Moon's appearance and position in the sky. The most obvious cycles are the sideral month (27.32 days) and the synodic month (29.53 days) mentioned in Phases of the Moon above. The sideral month is the period where the Moon completes exactly one 360° orbit around the Earth with respect to the fixed stars. The longer synodic month includes a component of the Earth's own orbit about the Sun.
Looking down on the North poles of the Earth and Moon, the Moon orbits the Earth, and the Earth orbits the Sun in the counter-clockwise direction.
The fourth cycle concerns the ecliptic, the path followed by Sun and planets through the sky. The Moon's orbit is inclined by 5° with respect to the plane of the ecliptic, which itself is inclined by 23.4° to the Earth's equator. The Moon crosses the ecliptic twice every 27.21 days (draconic month) at points called the ascending and descending nodes (i.e. South-to-North and North-to-South). As the draconic month is 0.11 days shorter than the sideral month, each crossing occurs ~1.8° to the West of the previous respective node. And so the lunar nodes precess around the ecliptic over an 18.6 year period. A consequence of the precession is that the maximum Northern and Southern declinations vary from 18.4° to 28.4° through the cycle. Once every 18.6 years the Moon achieves the maximum Northern and Southern declinations, placing the Moon unusually high and low in the sky through the month. Solar eclipses occur when the new Moon coincides with a node; lunar eclipses when full Moon occurs on or very near a node.
These four periodic cycles result in placing the relative geometry of the Sun, Earth and Moon in identical positions every 223 Synodic months ~ 239 anomalistic months ~ 242 draconic months, a period called the Saros = 6585.32 days (~18 years and 11 days). This time period is used to accurately predict the time and location of solar and lunar eclipses.
Libration: Glimpsing the far side of the Moon
Whilst the Moon's axial rotation rate is constant, its orbital velocity varies between a maximum at perigee and a minimum at apogee, causing the Moon to appear to swing from side to side as it completes an orbit cycle. This east-west displacement of up to ±7.9° is called libration in longitude. There is also libration in latitude as the Moon's equator is inclined to its plane of orbit by 6.4° - sometimes the North pole is in view, sometimes the South pole. Librations in longitude and latitude occur together and continually. A libration animation is posted on Wikipedia. Librations allow 59% of the Moon's surface to be seen over time, so it is possible to image features on the far side of the Moon, albeit at a shallow angle! An example is Mare Orientale. Strongly foreshortened structures that are close to the edge of the disk, such as giant crater Bailly and Mare Australe, better reveal themselves at favourable librations. A libration calculator can be found on the heavens-above website.
Foreshortening is the effect where features appear compressed in one axis due to the viewing angle. Lunar features are foreshortened due to the curvature of the Moon's surface; it increases dramatically towards the edge of the visible disc. The foreshortening across map grids A8 and O8 ranges from 50% to 100% at the Moon's edge, but at favourable libration foreshortening at the inner edge of A8 reduces to ~30%, whereas at unfavourable libration A8 vanishes to the far side. Foreshortening in the other regions can vary significantly due to libration: D8 averages 15% but can range from ~5% to ~25%.
The Moon is a small place, with a diameter of 3,476 km, and a circumference of 10,920 km. The Moon's surface area is just under 38 million km² so at any one time, we can see up to 19 million km². This area is similar to that of South America (17.8 million km²) or USA + Canada (19.8 million km²). The regions near the edge of the visible disc are however considerably foreshortened. Foreshortening is less pronounced within a disc that is 80% of the diameter of the lunar disc, encompassing 64% of the visible disc, but only 40% of the actual visible area. This area of ~7.6 million km² is equal to that of Australia or continental USA!
|Lunar Feature||Approximate Diameter||Approximate Area||Similar Sized Area on Earth|
|Mare Imbrium||1,145 km||1,000,000 km²||Egypt, South Australia|
|Mare Tranquillitatis||873 km||600,000 km²||France, Madagascar|
|Mare Serenitatis||674 km||350,000 km²||Japan, Germany|
|Mare Crisium||555 km||240,000 km²||New Zealand, Michigan-USA|
|Mare Nectaris||333 km||87,000 km²||Portugal, Mayne-USA|
|Plato (crater)||101 km||8,000 km²||Cyprus, Puerto Rico|
Did Buzz Aldrin visit the Moon?
Yes he did: Video
Atlases and References
Anyone interested in observing the Moon should have a copy of Antonin Rukl's Atlas of the Moon, published by Hamlyn. It contains 76 beautiful hand drawn tinted maps, 8 maps covering libration zones, 50 photographs of interesting features and a comprehensive index of named features. I have the 1991 edition; it was republished in 2007. If you prefer a software-based atlas, Virtual Moon Atlas v5.1 is a free Windows-based program, by Christian Legrand and Patrick Chevalley, that presents a detailed image of the Moon on your PC for any time and date. A recommended online atlas is LROC WAC mosaic of the lunar nearside. This impressive mosaic was constructed using ~1,300 images from the Lunar Reconnaissance Orbiter Camera. The maps present the Moon as seen from Earth and can be easily scaled. It shows resolution down to 0.3km. Though features are not named it is useful for comparing fields and images. It can also be downloaded as a 549 MB tiff file. I've never found Google Moon to be particularly useful.