It frequently happens that a person looking at the full moon through a telescope for the first time exclaims "Look at the north pole!" What prompts the exclamation is the view of Tycho and its rays as shown by Plates 5-2 and 8-3. It looks amazingly like the polar circle with radiating lines of longitude. Even an observer who knows perfectJy well that the poles of planets are not marked by any external signs and that meridians are artificial lines drawn for convenience on maps is struck by the similarity.
Tycho and Copernicus are competitors for pre-eminence among the craterlike features of the lunar surface. When the sun is low Copernicus definitely outclasses all other ringed plains, but when the sun is high Tycho is so magnificent that even the great maria scarcely rival it. Copernicus has the advantage of being situated in one of the dark, smooth maria. Tycho is in one of the roughest bright areas of the moon. Around it are great mountain-walled plains (Plates 8-1 and 8-2), some of them with diameters more than twice that of Tycho. When the sun is low the shadows cause these walled plains to stand out in majesty. The surfaces of nearly all the walled plains are composed of rock of one shade, so when the sun is high and shadows are short they almost disappear. But at such times Tycho and its brother ringed plains brighten far more than the surrounding areas, and divert our primary attention from the other features.
Plate 8-1. Sunrise on Tycho and Deslandres
In Plate 8-1 Tycho is exactly on the sunrise terminator. Only the crater rim is illuminated; all of the interior part, including the central mountain, is in darkness. Crater rims always are higher than are any central mountains that may exist. The rough features to the left of Tycho are emphasized by their long shadows. To the north of Tycho, toward the bottom of Plate 8-1, is Deslandres, the largest of all the mountain- walled plains, and one that is certainly older than most of the other craterlike enclosures on the moon. On this plate it is observed as one of the most conspicuous of all features, but its walls are so low that at even a day's phase later one must look carefully to find it. Plate 8-2 shows within Deslandres an intensely bright spot that is related to the Tychonian ray system. This spot is bright, however, even when it is near the terminator with the sun too low for Tycho's rays to be observed. It is called "Cassini's Bright Spot" because of the assertion by that astronomer, three centuries ago, that he had observed a cloud there and that a short while later a "new formation" first became visible. It is interesting that both Neison and Goodacre insisted mistakenly that the feature does not exist. A small crater (Plate 8-2) is easily observable throughout a good part of the lunar day. Undoubtedly Cassini was mistaken concerning the seventeenth-century origin of the object, but the later students must have made insufficient observations, and those at the wrong time of the month. This is a very easy error for even an experienced observer to make concerning features about which he has only casual interest.
Plate 8-8. Fading of Tycho's rays
The bright spot is useful in locating Deslandres when the sun is high. Plate 8-8, made in the afternoon, exhibits the craterlet almost as beautifully as does Plate 8-2, made in the morning.
Plate 8-2. Tycho and Deslandres in early morning
Plate 8-2 was made at a phase only a day later than that of the preceding picture, but a significant change has taken place. All of the floor of Tycho is illuminated, giving proof that, as in the case of Copernicus, its depth is small in comparison to its diameter. Among the mountain-walled plains Deslandres has become almost invisible and changes in others, such as Maginus, are noticeable. The point to remember is that the mountainous areas of the moon are in general less rugged than observations near the terminator would indicate. Tycho is much less deep in relation to its diameter than even a shallow pie pan, and the walled plains such as Maginus are still flatter. A man standing at the center of Clavius or of Ptolemaeus would have no idea that he was even within an enclosure because of his point of view.
Plate 8-3. Details of ray system near Tycho (Genovese)
On Plate 8-3, a few days after sunrise on Tycho, the whole area is
entirely unrecognizable. It is possible at this phase to make exposures
of negatives and photographic prints in such a way that to some extent
they restore to visibility the details of the central portion of Tycho,
but mere exposure cannot restore the lost mountain - walled plains,
which depend almost exclusively on shadows for their visibility. At first
glance Plate 8-3 shows Tycho as a circular white blob with great, almost
radial, bright spokes. It may be observed next that the image of Tycho
is scalloped, where craters and parts of craters exist on its rim. Immediately
outside the rim is a dark area into which the rays scarcely intrude. Neison
wrote of this area as a plateau but an examination of various photographs
indicates that it is more likely a slight depression. Nine of the ray spokes
exist if we consider the double one toward the lower right as two and also
include the rather indefinite one that bisects the great gap to the right.
Between the major rays, we find the same sort of elemental plume rays as
around Copernicus. Unlike the major rays, they are radial. The major ray
toward the lower left corner of Plate 8-3 is especially notable. Even a
casual examination will show that it ends before the edge of the picture
and is replaced by a closely parallel ray that starts about half-way between
Tycho and the edge.
Plate 8-4. Mt. Wilson and Palomar Observatories, 1958 August
1d 07h 22m UT, phase 15.53 days, colongitude 105 degrees. Kodak
IV-N plate with infrared filter for extreme contrast. Details of
Tycho's greatest left-hand ray.
Plate 8-4 has been made, by use of a high-contrast emulsion and proper choice of exposure time, to show the structure of this ray, which is primarily a series of bright craters. A fuller discussion of this ray belongs properly to a study of the nature of the lunar rays.
Plate 8-5. Mt. Wilson & Palomar Observatories, 1955 October 8d 12h 53m UT,
phase 22.27 days, colongitude 176 degrees. Southern area of the Moon at third quarter.
Plates 8-5 and 8-6 are enlargements of a photograph of Tycho and its surroundings made after the intensity of the great rays has decreased in the late afternoon sunshine. A partially sunken oval area breaks the wall slightly at the lower right- hand portion of the rim and therefore cannot be older than Tycho.
Plate 8-7, NASA link click here
Plate 8-9. Mt. Wilson & Palamar Observatories,
1958 July 9d 10h 40m UT, phase 22.11 days,
colongitude 185 degrees. Setting sun on Tycho.
Plate 8-9 advances the time to sunset, with the central peak casting its long shadow all the way to the inner left hand wall. Tycho is no more conspicuous than it was at sunrise, but during the day its radical changes in appearance have furnished data that are valuable for interpreting not only the origin of the violently explosive craters but also the lunar surface as a whole.
Section 64 - Tycho Directory, Lunar Chart
This web page was created by The Lunascan Project
The original hard copy and illustrations were written by Dinsmore
Alter and used in the 1967 version of PICTORIAL GUIDE TO THE MOON.