Observing the Moon

Moon is sometimes pleasantly introduced like the nearest celestial body to Earth. More simply, Moon is a fine object for an amateur to look at and to observe. Each month, Moon is appearing as a fine crescent on the western horizon soon after new moon. Moon increases then into a full quarter, and reaches full. At that moment, it is rising on the eastern horizon at the moment when Sun sets West. Moon eventually wanes down, through another quarter, to a mere last crescent. It then is rising few before Sun. All along its track in the sky, Moon is always providing observers with fine moments

Observational Basics

the Moon is an inspiring object in terms of observation. picture site 'Amateur Astronomy', from Celestia

Main observational trick about Moon is to observe lunar features when they are near what is termed the "terminator". Terminator is where the lighted parts of Moon are separated of the parts still in the dark by Sun's light. When Moon is waxing or waning Sun's light is grazing the Moon's surface along the terminator, enhancing the landscape's appearance. Some features -albeit rarer- are best seen at full moon however like the rays systems emanating from some craters. Lunar features mainly sort into mare, hills and mountains, and craters. Mare are mostly basalt flows which flooded lunar surface due to meteoritic shocks. Mare may be dated 3.8-3.1 billion years old. Hills and mountains are ancient highlands dating back to about 4 billion years. Craters were created by various impactors all along Moon's life. Such a bombardment began 4.6 billion years ago. Latest and polemical theory about Moon's origin is that it formed from a large impactor hitting Earth, ejecting material in orbit. This material eventually accreted into Moon

The time in days counted from the time of new moon is called the Moon's age as the concept of "percentage of Moon's surface illuminated" is the ratio of the area of Moon's near side illuminated to the total area, multiplied by 100. This value stars at 0 percent at the moment of the new moon to reach 100 percent at full moon, via the first quarter. Then the opposite, back to new moon. Technically, as far as phases are concerned, the four primary phases (new moon, first quarter, full moon, last quarter) are when the ecliptic longitude of the Moon (where it is along the ecliptic) is differing from that of Sun by 0, 90, 180, and 270° respectively. for more about Moon rotation, orbit, phases, see the tutorial "Moon's Phases"

Another remarkable Moon feature is the one due to what is termed "libration". Although Moon is always turning the same face to us, we are allowed to see about 59 percent of its surface in total. This is due to the fact that Moon' orbit is an ellipse, with a point nearest (perigee, from the Greek "peri", "near", and "geos", "Earth") and a point farthest (apogee) Earth. The Moon is speeding on its orbit at the perigee and slowing at the apogee as its rotation rate is remaining the same. Hence we see a little more than a strictly 100 percent near side, some percentage each side of it. Moon eastern and western parts but northern and southern are concerned too by a libration phenomenon, albeit less. This one is due to the Moon axis' tilt. Libration makes possible to see features usually out of reach. A well-known exemple are regions beyond Mare Crisium on the eastern limb or the famous Mare Orientale, beyond Grimaldi Crater. At last due to the perigee/apogee concept, Moon at the perigee (nearest Earth) has a larger apparent diameter, and a smaller one at the apogee (farthest Earth). This is not of great importance for observation however, albeit some are waiting for the most favorable perigees to make astrophotography at Moon. At the apogee Moon is 26,000 miles farther than at the perigee, which translates in a difference in apparent diameter of about 2.5' wide in average. At last, note that, as Earth itself has a nearest (perihelion; in January each year) and a farthest (aphelion; July) point from Sun, this means that, each year in winter, Moon is somehow brighter as it's nearest Sun altogether with Earth as it's dimmer during summer. The addition of perihelion/aphelion (Earth) and perigee/apogee (Moon) values makes that moonlight at full moon is varying by about 15 percent each side of an average value. Although this seems high, it mostly goes unnoticed as we have no point of reference when looking at Moon at such or such period of the year

Instruments, Techniques

Moon light does not need as much cares as does Sun's but it may be too harming. Appropriate filters provide more confortable views and more details at the same time

Moon is fine to view in various instruments. Binoculars or larger instruments used at small power are already providing a general view of the Moon, helping to learn landscape main features like the main seas, craters, or highlands. Larger binoculars or small instruments provides closer views of regions of interest. Larger instruments allows fine detailed views. Drawing sketches is training and deepening observer's skills. Moon may be observed naked-eye too. Moon is a fine and easy object for astrophotography

see an observation-minded basic Moon map