By Alan Hale
For the Daily News
Just as it has done for almost uncountable millennia in the past, and will continue to do for many millennia into the future, the moon will go through its regular cycle of phases this month as it travels on its orbit around Earth. Indeed, these phases are entirely due to that orbital motion;while half of the moon is always lit by the sun, as it makes its way along its orbit we here on Earth see varying percentages of that sunlit half. We will see all of it on Tuesday night going into Wednesday morning, Oct. 7 and 8, when we will be seeing the moon in its full phase. A little over two weeks later, on Oct. 23, the moon will be between the Earth and the sun and we won’t see any of that sunlit half; this is the phase we call a new moon.
If the moon’s orbit around Earth were in the same plane as the Earth’s orbit around the sun, at every new moon the moon would pass directly between the Earth and the sun, we would have a solar eclipse. If at every full moon, the moon passed through Earth’s shadow, we would have a lunar eclipse. However, the moon’s orbit is tilted some 5 degrees with respect to the Earth’s orbit, and as a result most of the time the moon passes well north or south of the sun at new moon, and well north or south of the Earth’s shadow at full moon. Only on those occasions when the moon is crossing the plane of the Earth’s orbit at the same time it is at new moon or full moon do we have an eclipse, and these occasions occur at approximately six-month intervals.
It so happens that, during October, we are in one of these semi-annual eclipse seasons. One of the results of this is that, on this coming Wednesday morning, the moon passes through the Earth’s shadow and we will experience a total eclipse. From here in New Mexico the moon begins entering the Earth’s dark inner shadow, or umbra, at 3:15 a.m. MDT. (For about half an hour or so before that we may see some dusky shading on the moon as it travels through Earth’s outer shadow, or penumbra.) The moon is fully immersed within the umbra at 4:25 a.m. and remains there for almost exactly one hour; afterwards, it begins exiting the umbra, and completes that exit at 6:34 a.m.
Although the moon will spend an hour completely immersed in Earth’s umbra, it will not be completely dark. The Earth’s atmosphere refracts, or bends, sunlight, and consequently the moon takes on a reddish or orange hue during totality. In essence, this is the reflection of all the sunrises and sunsets taking place on Earth during that time. If there is a lot of dust in the atmosphere, for example, as a result of a massive volcanic eruption, the moon can become quite dark during a total eclipse; while there have been a few recent eruptions here on Earth, there hasn’t been anything on quite that scale, so the moon will probably not get that dark this time.
At new moon two weeks later on the 23rd, the moon passes directly between the Earth and the sun and we have a solar eclipse. The moon’s umbra misses Earth entirely and thus there isn’t a total eclipse anywhere, however the moon’s penumbra will cover a fairly large area of the Earth’s surface which will accordingly experience a partial eclipse. Almost all of North America (except for the southern regions of Central America), as well as far eastern Russia, will see this eclipse.
From here in New Mexico, the eclipse will begin shortly after 3:30 p.m., will be at its maximum slightly over an hour later, and ends just over an hour after that. Near the middle of the eclipse approximately 45 percent of the sun’s diameter will be covered up by the moon.
Since a significant amount of the sun’s surface will still be shining even during the deepest part of the eclipse, no one should look directly at the sun during that event (just as is true at any other time except during a total solar eclipse). The safest way to view the eclipse is to use binoculars or a telescope to project an image of the sun onto a light-colored surface that is in shade.
This month’s lunar eclipse is part of a somewhat rare sequence of lunar eclipses known as a tetrad. Another total lunar eclipse took place six months ago, on April 15, an event which was also visible from New Mexico. During the next eclipse season a third total eclipse takes place on the morning of April 4, 2015 (although totality will only last briefly for 5 minutes), and meanwhile the fourth, and last, eclipse of this tetrad takes place on the early evening of Sept. 27, 2015. Both 2015 eclipses will also be visible from New Mexico, which puts us in a most fortunate situation in being able to view all four eclipses of this tetrad sequence.
Solar eclipses will also be taking place at the respective eclipse seasons, but none of these events are visible from New Mexico. The next solar eclipse visible from here takes place a little less than three years from now, on Aug. 21, 2017. From here in southern New Mexico this will be a deep partial eclipse, with about 70 percent of the sun’s diameter being covered up by the moon, but along a narrow strip of land which stretches from northwestern Oregon across to South Carolina the moon will completely cover up the sun, producing a total eclipse that at its longest, near St. Louis, will last slightly over 2 minutes 40 seconds. This will be the first total solar eclipse to cross the continental United States since 1979, and should be an event very much worth waiting for.