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As referred earlier, even a casual observer will notice the cyclic phenomena that dramatically change the Moon's appearance in the sky. The periodicity of such changes is about 29 1/2 days, and that period is called lunation or synodic month. From time to time, two spectacular events may also take place at specific time-points of the lunation, namely solar and lunar eclipses.
During a lunation, the terminator, that is the boundary between the illuminated and non-illuminated sections of the lunar disk, will travel twice from one limb to the other. Starting at new-moon, time at which our satellite would appear 0% illuminated if we could observe it from our vantage point (new-moon occurs during the day, and is usually invisible due to the sunlight scattered by the atmosphere), the morning terminator will become visible at the eastern limb, will proceed westwards crossing the center of the Moon's disk at first-quarter, and will further progress until it reaches the western limb at full-moon. During this first half of the lunation, the percentage of the Moon's visible hemisphere that is seen illuminated by the Sun will gradually increase, reaching 50% at the first-quarter and 100% at full-moon. The second half of the lunation starts at full-moon and is characterized by the appearance of the evening terminator at the eastern limb. From this point up to the next new-moon, we will recapitulate the gradual east-west movement of the terminator witnessed earlier. However, we will now notice a steady decrease of the percentage of the Moon's near-side that is seen illuminated, namely 50% an last-quarter down to 0% at new-moon. The below left mosaic shows the typical appearance of the Moon on days 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23 and 25 of the lunation. The new-moon is usually invisible from Earth, except during solar eclipses, as will be explained in the next part of this section.
One of the observational projects that is popular among amateurs is to spot a very "young" or "baby-Moon" like that shown in the above right image. It can be a real challenge if less than one day has elapsed since the previous new-moon, if atmospheric transparency is not adequate, or if the plane of the Moon's orbit is low with respect to the horizon. Since the sky will still be fairly bright while the Moon is above the horizon, a clear view to the west will be a major help, as will be the use of binoculars and the previous knowledge of the Moon's position.
Another quite interesting project that everybody should try, and that I strongly recommend, is to follow the appearance of a given lunar formation throughout the lunation. In the example depicted in the above image, the crater Eratosthenes (yellow arrow) shows profound changes in a 7-day time-lapse, specifically from first-quarter to full-moon. From a quite distinct object when near the morning terminator (top-left frame), it practically disappears at full-moon (bottom-right frame). As you may have guessed, the opposite effect will occur from full-moon to last-quarter. Opposite, also because of the complementary orientation of the shadow effects that will progressively appear. But the almost shadow-free conditions that take place near or at full-moon also deserve the observer's attention. For instance, lunar rays will become prominent, as will the bright craters from which they originate. Also in the above image, the changes that occur in the Copernicus crater/ray system (located at the lower-left corner of all frames) are obvious. In addition, a number of small craters that are inconspicuous when observed at low-angle illumination, will become very bright at full-moon. The contrast between dark maria and bright terrae, namely the southern highlands, will also increase at full-moon, and craters flooded with mare material, like Plato and Grimaldi, will present dark floors.
When it is referred that a lunation lasts about 29 1/2 days, absolute beginners almost immediately think about modifications that occur at a daily time-scale, that is changes that only take place in the Moon's appearance from one day to the other. Although a 24-hour period is more than enough to produce obvious changes, the truth is that these changes occur continuously, and can be spotted in a single night if we look at the right places. An the right places are those at the terminator, particularly if one or more major lunar formations are present in the field-of-view. In the above time-lapse animation, obtained in just a few hours of a single observation session, the terminator is moving continuously from right to left (east to west). As a consequence, the length of the shadows cast by the Apenninus Montes steadily decreases, and the rim of crater Eratosthenes is seen progressively more illuminated.
The twelve images above are representative of the modifications that take place in the Moon's near-side during the first half of the lunation (images illustrate the changes that occur from days 3 to 14 after new-moon). I invite you to compare what happens to the appearance of a given lunar region as days go by. There is no need to know in advance the names of any of the maria, mountains or craters of the Moon. Just look for differences in visibility, distance to the terminator, shadow length and brightness, and/or association with rays. Then, why don't you try the "live" experience of looking at the Moon through a telescope? I'll bet you will like it!
And after following an entire lunation is there anything else new to see in the Moon? The answer is YES..., the next lunation..., and the next... The reason is quite simple. Even if seeing conditions were favorable in all the observation sessions, and if the observer had time and memory to grab all the visual information that is available, the fact is that each lunation is unique! First of all, remember that the progression of the terminator is continuous and that the lunation lasts about 29 1/2 days. This factor alone determines that what is seen in day 7 of a given lunation will be considerably different from what will be visible on day 7 of the following lunation (about 12 hours "different"). But there are other factors. Perturbations are always occurring in the Moon's orbit, and a very interesting phenomenon called libration also occurs in each lunation Together, these additional factors determine that in a given phase (or day of the lunation) the angle of illumination of a particular formation will always be different. Thus, a different "moonscape" is always waiting for our observation!
What causes lunations, librations, and also eclipses to occur is the subject of the next part of this section.
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© A.Cidadão (1999)
