Two total lunar eclipses occur this year, on April 4th and September 27−28. Meanwhile, a total solar eclipse in March sweeps across remote Arctic waters on March 20th, and a partial event on September 13th is likewise poorly placed for observing.
Any list of nature's grandest spectacles would certainly include eclipses of the Sun and Moon. Up to seven of them can take place in one year, though the last time that happened was 1982. The fewest possible is four, as will be the case in 2015. Neither of the solar eclipses — one total and one partial — is observable from the Americas. Both lunar eclipses are total; April's favors the West Coast, while September's is best on the East Coast.
Why Do Eclipses Happen?
A solar eclipse, such as the one pictured at right, occurs only at new Moon, when the lunar disk passes directly between us and the Sun. Conversely, a lunar eclipse takes place during full Moon, when our satellite passes through Earth's shadow. These alignments don't happen at every new and full Moon because the lunar orbit is tipped about 5° to Earth's orbital plane — only occasionally do the Sun, Earth, and Moon line up exactly enough for an eclipse to occur. (The technical name for that, by the way, is syzygy.)
Three types of lunar eclipse are possible (total, partial, and penumbral) depending on how deeply the full Moon plunges into or near the umbra, our planet's dark, central shadow. If it goes all the way in, we see a total lunar eclipse that's preceded and followed by partial phases. If the Moon skims part way into the umbra, only the partial phases occur. And if its disk passes just outside the umbra, it still encounters the weak penumbral shadow cast by Earth.
Interestingly, this year's two lunar events mark the conclusion of a series of four consecutive total lunar eclipses in 2014–15! Such eclipse tetrads are not common — the last one occurred a decade ago, but the next won't begin until 2032.
Fortunately, every lunar eclipse is observable anywhere on Earth where the Moon is above the horizon. (But there's still an element of luck involved: as described below, April's total lunar eclipse promises to be gorgeous from Los Angeles but completely unobservable from New York.)
However, solar eclipses more tightly restrict where you can see them because the Moon casts a smaller shadow than Earth does. If the Moon completely hides the Sun, the eclipse is considered total. With its brilliant disk completely covered, the Sun's ghostly white outer atmosphere is momentarily revealed for durations from seconds to several minutes. In November 2013, for example, planeloads of eclipse-chasers converged in a remote portion of northern Kenya to watch just 11 seconds of totality.
A completely eclipsed Sun can be viewed only from a narrow track or path on Earth's surface that's typically just 100 miles (160 km) wide. Outside of that path, about half of the daylit hemisphere of Earth is able to watch a partial eclipse as the Moon obscures a portion of the Sun.
Occasionally the Moon passes directly in front of the Sun but doesn't completely cover it. This circumstance is known as an annular eclipse, so-called because you can see a ring, or annulus, of sunlight surrounding the lunar disk. But an annular's path is likewise narrow, and outside of it observers see only a partial cover-up.
The Four Eclipses in 2015
Below are brief descriptions of this year's eclipses of the Sun and Moon. You'll find more details for March's total solar eclipse and the two lunar eclipses both on this website and in Sky & Telescope magazine as the date of each draws near. Times are in Universal Time (UT) except as noted.
March 20: Total Solar Eclipse
Whenever the Moon covers the Sun, the narrow path of totality can be anywhere in the world — often traversing remote locations. The path of this year's total solar eclipse is doubly challenging — both to get to and to see once there. It's an especially wide track, up to 303 miles (487 km) wide, that offers a maximum of 2 minutes, 47 seconds of totality. But that's little consolation because the path is largely confined to the extreme North Atlantic and the open water between Greenland and Scandinavia. (Fun fact: the Moon's shadow leaves Earth as it crosses the North Pole!)
Totality will only be seen from the remote Faroe Islands (halfway between Iceland and Norway) and Svalbard (halfway between Norway and the North Pole). In these locations, diehard eclipse-chasers can expect 2 minutes of totality beginning at 9:41 UT and 2½ minutes beginning at 10:11 UT, respectively. These ice-swept outposts are so far north that in most of Longyearbyen, on the island of Spitsbergen, mountains will block views of the eclipsed Sun. Meanwhile, the prospects for clear skies from those bits of dry land — and for the tracts of sea around them — is relatively poor, with no site offering better than a 50:50 chance of clear sky at eclipse time.
Because the Moon is near perigee and thus look somewhat larger than usual in the sky, the area from which a partially covered Sun can be seen is also large: all of Europe, northwestern Asia, and northern Africa.
April 4: Total Lunar Eclipse
It's not uncommon for a solar eclipse (new Moon) to be followed two weeks later by a lunar eclipse (full Moon), or vice versa. But it's rare that two total eclipses pair so closely, and that's the case this year. (The last time this occurred was in 2003, and the next will be in 2033.)
However, April 4th's event only barely qualifies as a total lunar eclipse. The Moon is completely immersed in Earth's dark umbral shadow for just 4½ minutes, centered on 12:00 UT. In fact, because the edge of totality is a fuzzy boundary, totality's duration depends on what definition for "umbra" is you adopt. For example, the U.S. Naval Observatory’s calculations (used by Sky & Telescope) yield a duration of 12.3 minutes, while Fred Espenak’s Fifty-Year Canon of Lunar Eclipses says 8.6 minutes.
Regardless, the event's timing favors the West Coast (mid-eclipse is at 5:00 a.m. Pacific Daylight Time). Anyone located east of the Mississippi gets to glimpse only the early partial phases before the Moon sets in brightening morning twilight. Because of the barely-there nature of this umbral crossing, the Moon's northern half (being closest to the umbral boundary) is certain to look brighter than its southern half. The partial phases begin at 10:16 UT and end at 13:45 UT.
September 13: Partial Solar Eclipse
Here's an eclipse that few can hope to see — unless you happen to be visiting Antarctica. Unlike March's solar eclipse, this time the Moon is relatively far from Earth, thus limiting the regional extent of visibility. The greatest coverage (79% of the Sun's diameter) occurs near Germany's Neumayer Station on the coast of Antarctica. As shown in this map, skywatchers in southern Africa can see a modest bite in the solar disk (up to 38% of its diameter) just after sunrise.
September 27–28: Total Lunar Eclipse
Observers throughout the Americas, as well as in western Europe and Africa, have the best seats for the year's final eclipse. Mid-eclipse is at 2:47 UT on the 28th, which corresponds to the evening of the 27th across the U.S. and Canada.
The total phase of this eclipse lasts from 10:11 to 11:23 p.m. EDT on the East Coast, where the Moon appears high up in the constellation Pisces. On the West Coast, totality runs from 7:11 to 8:23 p.m. PDT and begins with the darkened lunar disk low in the eastern sky in deep evening twilight. The partial eclipse begins at 9:07 p.m. EDT and ends at 12:27 a.m. EDT on the 28th. (Early phases of the eclipse occur before moonrise in western North America.)
Unlike the barely total event in April, this time the Moon plunges deeply into the southern half of the umbra. Consequently, the upper (northern) half of the lunar disk should be much darker than the lower half. The Moon is also at perigee, so the lunar disk appears about 13% wider than it did in April.
By the way, if you want to do more than simply gaze at the eclipsed Moon, several useful observing projects can enhance your experience.
Looking ahead, 2016 will again feature only four eclipses. But next year the emphasis switches: there'll be a total solar eclipse on March 9th (visible from Indonesia) and an annular eclipse on September 1st. The only lunar eclipses are penumbral, meaning the Moon grazes through Earth's outer shadow and never really gets dark, on March 23rd and September 16th.