Regular readers of this column may have come to the conclusion that there is nothing in the spring sky besides the Virgo Cluster and a few scattered individual galaxies. This probably reflects, more than anything else, the prejudice of your correspondent, who has spent most of his career studying galaxies, in one form or another. He asks once more for your indulgence.
The distribution of galaxies is a question of great importance in cosmology, since it probably reflects the distribution of matter at a very early stage in the evolution of the universe. Although appearances can be misleading, the first thing that strikes us about the distribution of galaxies is their tendency to occur in clusters. Abell studied the Palomar Sky Survey and listed almost 3000 clusters, and these were only the bright ones. He classified them according to richness, which is a measure of the number of galaxies brighter than a certain limit and within a certain radius of the center. Another of the many ways of classifying clusters is the Bautz-Morgan class, which is based on the degree to which a single bright galaxy dominates the cluster.
Examples of all of the different types of clusters can be found in the spring sky. Although when seen visually these are never spectacular objects, the brightest galaxies can be found easily enough with a moderate sized telescope.
The Virgo Cluster is located on the northern edge of Virgo, 1 hour 30 minutes west of Arcturus, and a few degrees south, or 1 hour due east of Denebola. Although it contains a number of bright galaxies, it is not a particularly rich cluster. Its brightest, and certainly most exciting, member is the active elliptical galaxy Messier 87, but it is not so much brighter than the others as to be called dominant. Actually, most of the bright galaxies in the Virgo Cluster are spirals, and this is typical of the less concentrated clusters.
The Coma Cluster, located near the northern edge of Coma Berenices, is about 15 degrees north and a bit west of the Virgo Cluster. It is 1 hour 20 minutes west and 9 degrees north of Arcturus and quite close to the north galactic pole. It is much more distant than the Virgo Cluster, so even the brightest galaxies may be hard to observe. It is much more concentrated than the Virgo Cluster, and a large fraction of its members are elliptical galaxies. It is dominated by the bright elliptical galaxy NGC 4886, putting it in the intermediate Bautz-Morgan class II. The reasons for these differences between clusters might be the subject of another essay.
The Hercules Cluster, on the very western edge of the constellation, is also of interest. It is located about 7 degrees west-southwest of Kornephoros and 10 degrees southeast of Alphecca. It is even more distant than the Coma Cluster, but in both clusters the brightest galaxies are about magnitude 13. This is a good example of a multiple cluster, with several concentrations close to one another. The secondary concentrations here all lie 1 or 2 degrees to the south of the main one. They are less dense and their galaxies are not so bright, but they all appear to be at about the same distance.
There are many other such groups of clusters, suggesting ever larger structures in the Universe. Large-scale maps of the distribution of galaxies suggest that these structures might best be described as large regions of low density, which have come to be called voids, surrounded by sheets or filaments in which the density is higher. Once such a region begins to form, the remaining matter is attracted to its edges by the gravitational force of the matter already located there, and the density contrast increases. This is where galaxies will form, and the intersections of these sheets of matter will appear as clusters or groups of clusters. In this way we can deduce something about the distribution of matter in the early universe, during a period earlier than any that we might hope to observe directly. We might also learn something about the conditions under which galaxies will begin to form.
In the middle of April Mercury will be easily visible high in the evening sky, but by the end of the month it will have faded and
vanished into the twilight. Its reappearance in the morning sky in early June will be observable only from the southern hemisphere. During the night of May 6-7 (our time) Mercury will transit the disk of the Sun. This unusual event, which occurs only about once every 8 years, will be unobservable from most of North America. Observations of transits of Mercury and Venus used to be very important to celestial mechanics, but they have now been largely superseded by more accurate methods of position determination.
Venus will be very low in the morning sky at the beginning of the quarter, and observing conditions will only get worse as the quarter progresses. It will not be easily observable again until October.
Mars rises after midnight as the quarter begins, and earlier as summer approaches. It is far south at the beginning of the quarter and, although it gradually moves northward, it remains unfavorably placed for observation.
Jupiter has passed opposition and will be stationary on April 4. It will be quite far north and well placed for observation in the evening sky during April and May.
Saturn is still easily observable in the evening sky during April, but thereafter it begins to vanish into the twilight. It is in conjunction with the Sun on June 24.
There will be four meteor showers of interest during the spring quarter, three of them close to the end of April.
The Lyrids may be observed in the interval April 16-25, with a possible peak on April 22. Their behavior has been irregular, with occasional outbursts of activity. They are best observed in the middle of the night, between the rise of the radiant and moonrise.
The pi-Puppids are a young stream, whose parent comet, 26P/Grigg-Skjellerup, is close to perihelion, so we may expect enhanced activity. They are best observed from the southern hemisphere. A sharp peak might be expected on April 24, and they should be looked for during the evening hours.
The eta-Aquarids are interesting because of their association with comet 1P/Halley. They might be observed from the middle of April through the end of May, with one or more peaks on or around May 6. They can be seen in the morning hours, when the Moon has long since set. Again, the southern hemisphere is more favorable. This shower is known for its tendency to produce bright meteors with persistent trails.
The June Bootids were long thought to be lost, but they suddenly and dramatically reappeared in 1998. The parent comet, 7P/Pons-Winnecke is now near perihelion, but its orbit remains well outside that of the Earth, so we may have encountered a detached part of the stream, which has been perturbed by a resonance with Jupiter. A new encounter is expected in 2004, so we may already see an increase in activity this year.
Comet 2002 X5 (Kudo-Fujikawa) is fading rapidly, but it may still be visible with binoculars when it reappears in the evening sky toward the end of March.
Comet 2002 Y1 (Juels-Holvorcem) will pass perihelion, at 0.7 a.u., in April, when it should reach sixth magnitude. It will be large and diffuse, but well placed for observation in the northern morning sky.
Comet 2002 T7 (LINEAR) is still far from perihelion, which it will reach in April 2004. It will then be quite bright but visible mainly from the southern hemisphere. It is still very faint, but it is mentioned here because its significantly hyperbolic orbit makes it an object of particular interest.
A total lunar eclipse on the evening of May 15 will be visible from most of North America. Totality will end shortly after moonrise, in our neighborhood. The Moon will be close to perigee at the time, and thus, being also close to horizon, will look uncommonly large.
During the evening (our time) of May 30 there will be an annular eclipse of the Sun, visible only from the far north Atlantic. The Sun will be low in the northeast and the local time will be close to midnight! Due to the unusual geometry, the path of the eclipse will move from east to west. The region of annularity crosses the ocean with great speed, starting in Scotland and ending in the Davis Strait only 46 minutes later. Unfortunately we will see nothing of this eclipse from our location.