Thanks to the lunar samples bright back by Armstrong, Aldrin, Collins, and the other Apollo astronauts, it is well established that the vast majority of craters are impact craters formed when leftover debris from the formation of the solar system slammed into the Moon. But if we look carefully, scattered among all of those impact scars is direct evidence that the early Moon was also a hotbed of volcanic activity.

Some of the most intriguing evidence of that activity is the so-called lunar domes. Experts tell us that the lava that flooded the giant impact basins to form the maria we see today had a low viscosity. In other words, it flowed quickly due to its high temperature. As the lunar core cooled over time, however, the erupting lava decreased in flow rate as well as temperature, which increased its viscosity. As it continued to percolate through vents, the lunar lava's ability to flow away decreased, creating in the process low shield-like volcanoes, the lunar domes.

Most lunar domes are clustered together in small groups and lie in or are immediately adjacent to maria. A typical dome measures between 3 and 12 miles (5 to 19 km) in diameter, appears round or elliptical in shape, has an average slope of only 2° to 5°. Due to this gentle grade, volcanic domes are only readily visible right after sun-up. All quickly fade from view after the Sun moves higher in their sky. But when sunlight just grazes their tops, domes can appear quite striking.

At first pass, the crater Hortensius appears to be of no special significance. Just another impact crater among the myriad, right? Closer inspection shows that, although Hortensius itself is rather bland, its immediate surroundings are anything but. Visit here just as the light from the rising Sun grazes the adjacent plain of Mare Insularum (the Sea of Isles) and as many of 6 unusual mounds, or bumps, can be seen to the crater's northeast. The Hortensius dome field, as it is most often called, forms Moon's best known region of lava domes.

Although it only spans 9 miles from edge to edge, Hortensius is easy to pinpoint thanks to two prominent craters on either side, Copernicus to its (lunar) east and Kepler to its (lunar) west. Since Kepler still lies in darkness during the dome field's prime viewing time, we will need to rely on Copernicus as our guide. Hortensius resides just beyond the southwestern edge of Copernicus' pronounced ray pattern, or ejecta field, and forms a not-quite equilateral triangle with the larger crater Reinhold, also to Copernicus's southwest.

Once you've identified Hortensius, switch to between 150x and 200x, or higher if conditions permit, and focus your attention just to its northeast. Can you spot 5 or 6 "bumps" in the otherwise flat plain? If so, look carefully and you should also see that five of the domes are punctuated by tiny craterlets -- volcanic vents known as calederas -- centered on their summits. The vent of the sixth dome must have been concealed by lava.

If you enjoyed searching for the Hortensius domes, try your luck with another dome near the crater Milichius, also in Mare Insularum. Known as Milichius Pi, this dome measures about 7.5 miles (12 km) in diameter. Can you also spot its central caldera?

Remember, timing is critical. The best phases to view these are the day after the Sun rises over Copernicus and the day after it sets. Those are the 10-day old waxing gibbous and 24- and 25-day-old waning crescent phases, respectively. Both position the Sun low in the domes' sky, maximizing the visibility of their gentle topography.

Good luck with this month's challenge! And be sure to post your results in this column's discussion forum.

Remember that half of the fun is the thrill of the chase.  Game on!