A solar eclipse is an astronomical phenomenon, in which the sun is obscured by the moon. Total eclipses, in which darkness falls and the sun’s normally invisible atmosphere is seen around a blackened sun, attract many travellers.
Understand
A solar eclipse occurs when a portion of the Earth is engulfed in a shadow cast by the Moon which fully or partially blocks (“occults”) sunlight. This occurs when the Sun, Moon and Earth are aligned. Such alignment coincides with a new moon (syzygy) indicating the Moon is closest to the ecliptic plane. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.
If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every new moon. However, since the Moon’s orbit is tilted at more than 5 degrees to the Earth’s orbit around the Sun, its shadow usually misses Earth. A solar eclipse can only occur when the moon is close enough to the ecliptic plane during a new moon. Special conditions must occur for the two events to coincide because the Moon’s orbit crosses the ecliptic at its orbital nodes twice every draconic month (27.212220 days) while a new moon occurs one every synodic month (29.530587981 days). Solar (and lunar) eclipses therefore happen only during eclipse seasons resulting in at least two, and up to five, solar eclipses each year; no more than two of which can be total eclipses.
Total eclipses are rare because the timing of the new moon within the eclipse season needs to be more exact for an alignment between the observer (on Earth) and the centers of the Sun and Moon. In addition, the elliptical orbit of the Moon often takes it far enough away from Earth that its apparent size is not large enough to block the Sun entirely. Total solar eclipses are rare at any particular location because totality exists only along a narrow path on the Earth’s surface traced by the Moon’s full shadow or umbra.
An eclipse is a natural phenomenon. However, in some ancient and modern cultures, solar eclipses were attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.
Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse. It is technically safe to view only the total phase of a total solar eclipse with the unaided eye and without protection; however, this is a dangerous practice, as most people are not trained to recognize the phases of an eclipse, which can span over two hours while the total phase can only last a maximum of 7.5 minutes for any one location. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses.
Types
There are several types of solar eclipse:
total, in which the normally visible parts of the sun are totally obscured, causing night-like darkness to fall for several minutes, and the corona — the sun’s normally invisible atmosphere — to be seen radiating around the black circle of the moon
annular, in which the moon obscures the centre of the sun, causing a bright ring to be seen around the dark moon
hybrid, in which at various points along the eclipse’s path either an annular or total eclipse is seen
partial, in which a fraction of the sun’s surface is obscured by part of the moon
Total eclipses are the most dramatic solar eclipse, a very strange and beautiful spectacle. A total solar eclipse occurs somewhere on Earth once or twice most years but is only visible from a narrow ribbon of the surface — the eclipse’s path — with a partial eclipse experienced in a wider area. Partial eclipses are the least dramatic: unless one is viewing the eclipse deliberately it often just appears to be a somewhat dim day, as if overcast.
Annular and total eclipses begin with a partial eclipse in which progressively more of the sun is eclipsed until the maximum eclipse, called totality for total eclipses. The partial eclipse may last one hour or more either side of totality; totality is only a few minutes in length or less.
Total eclipses, like many orbital phenomena, can be predicted accurately very far in advance, and attract many travellers anticipating the sight. They can be easily seen and appreciated with the naked eye, but amateur astronomers frequently travel to them with telescopes. Some travellers, umbraphiles, make it a priority to travel to see as many eclipses as possible.
Viewing a total solar eclipse is a chancy thing: simple local cloud cover changes it from a eclipse viewing to a momentarily dark cloudy day!
The Moon’s orbit around the Earth is slightly elliptical, as is the Earth’s orbit around the Sun. The apparent sizes of the Sun and Moon therefore vary. The magnitude of an eclipse is the ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse. An eclipse that occurs when the Moon is near its closest distance to Earth (i.e., near its perigee) can be a total eclipse because the Moon will appear to be large enough to completely cover the Sun’s bright disk or photosphere; a total eclipse has a magnitude greater than or equal to 1.000. Conversely, an eclipse that occurs when the Moon is near its farthest distance from Earth (i.e., near its apogee) can only be an annular eclipse because the Moon will appear to be slightly smaller than the Sun; the magnitude of an annular eclipse is less than 1. Slightly more solar eclipses are annular than total because, on average, the Moon lies too far from Earth to cover the Sun completely.
A hybrid eclipse occurs when the magnitude of an eclipse changes during the event from less to greater than one, so the eclipse appears to be total at locations nearer the midpoint, and annular at other locations nearer the beginning and end, since the sides of the Earth are slightly further away from the Moon. These eclipses are extremely narrow in their path width and extremely short in their duration at any point, at most just a few seconds at any location within just a few kilometres of the centerline of the path. Like a focal point, the width and duration of totality and annularity are near zero at the points where the changes between the two occur.
Because the Earth’s orbit around the Sun is also elliptical, the Earth’s distance from the Sun similarly varies throughout the year. This affects the apparent size of the Sun in the same way, but not as much as does the Moon’s varying distance from Earth. When Earth approaches its farthest distance from the Sun in early July, a total eclipse is somewhat more likely, whereas conditions favour an annular eclipse when Earth approaches its closest distance to the Sun in early January.
Reason
Why do eclipses happen?
Solar eclipses happen when the moon is directly in between the earth and the sun. Another way to think about it is that the moon can cast a shadow, and if you’re in that shadow, it’ll look like the moon is covering up the sun or part of it. That’s an eclipse.
Why isn’t there a solar eclipse every month?
There would be if the moon’s orbit around the earth was in the same plane as the earth’s orbit around the sun. Instead, the moon’s orbit is tilted, so its shadow doesn’t usually hit the earth, which means that eclipses aren’t usually visible from the earth’s surface. Everything lines up just right for an eclipse only 2–5 times a year, and a total eclipse is only possible at most twice in a given year.
Why are some solar eclipses total while others are annular?
Both the moon and the sun vary in their distance from the earth. As a result, sometimes the moon looks slightly smaller than the sun (as in an annular eclipse), and sometimes it looks the same size or slightly larger (as in a total eclipse).
Do total solar eclipses happen on other planets like they do on Earth?
Yes, on some planets, but not very many. Earth has the lucky coincidence of a sun and moon that look roughly the same size to a viewer on the surface. The only other planet in our solar system with similar eclipses is Saturn, and due to the shapes and orbits of its moons, the eclipses there wouldn’t be as spectacular as they are on Earth.
Why do the sun and the moon look roughly the same size to a viewer on Earth’s surface?
The sun is 400 times the diameter of the moon, but it also happens to be roughly 400 times as far away.
Viewing
Looking directly at the photosphere of the Sun (the bright disk of the Sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring.
Under normal conditions, the Sun is so bright that it is difficult to stare at it directly. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare at it. Looking at the Sun during an eclipse is as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun’s disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse). Viewing the Sun’s disk through any kind of optical aid (binoculars, a telescope, or even an optical camera viewfinder) is extremely hazardous and can cause irreversible eye damage within a fraction of a second.
Partial and annular eclipses
Viewing the Sun during partial and annular eclipses (and during total eclipses outside the brief period of totality) requires special eye protection, or indirect viewing methods if eye damage is to be avoided. The Sun’s disk can be viewed using appropriate filtration to block the harmful part of the Sun’s radiation. Sunglasses do not make viewing the Sun safe. Only properly designed and certified solar filters should be used for direct viewing of the Sun’s disk. Especially, self-made filters using common objects such as a floppy disk removed from its case, a Compact Disc, a black colour slide film, smoked glass, etc. must be avoided.
The safest way to view the Sun’s disk is by indirect projection. This can be done by projecting an image of the disk onto a white piece of paper or card using a pair of binoculars (with one of the lenses covered), a telescope, or another piece of cardboard with a small hole in it (about 1 mm diameter), often called a pinhole camera. The projected image of the Sun can then be safely viewed; this technique can be used to observe sunspots, as well as eclipses. Care must be taken, however, to ensure that no one looks through the projector (telescope, pinhole, etc.) directly. Viewing the Sun’s disk on a video display screen (provided by a video camera or digital camera) is safe, although the camera itself may be damaged by direct exposure to the Sun. The optical viewfinders provided with some video and digital cameras are not safe. Securely mounting #14 welder’s glass in front of the lens and viewfinder protects the equipment and makes viewing possible. Professional workmanship is essential because of the dire consequences any gaps or detaching mountings will have. In the partial eclipse path, one will not be able to see the corona or nearly complete darkening of the sky. However, depending on how much of the Sun’s disk is obscured, some darkening may be noticeable. If three-quarters or more of the Sun is obscured, then an effect can be observed by which the daylight appears to be dim, as if the sky were overcast, yet objects still cast sharp shadows.
Totality
When the shrinking visible part of the photosphere becomes very small, Baily’s beads will occur. These are caused by the sunlight still being able to reach the Earth through lunar valleys. Totality then begins with the diamond ring effect, the last bright flash of sunlight.
It is safe to observe the total phase of a solar eclipse directly only when the Sun’s photosphere is completely covered by the Moon, and not before or after totality. During this period, the Sun is too dim to be seen through filters. The Sun’s faint corona will be visible, and the chromosphere, solar prominences, and possibly even a solar flare may be seen. At the end of totality, the same effects will occur in reverse order, and on the opposite side of the Moon.
Photography
Photographing an eclipse is possible with fairly common camera equipment. In order for the disk of the Sun/Moon to be easily visible, a fairly high magnification long focus lens is needed (at least 200 mm for a 35 mm camera), and for the disk to fill most of the frame, a longer lens is needed (over 500 mm). As with viewing the Sun directly, looking at it through the optical viewfinder of a camera can produce damage to the retina, so care is recommended. Solar filters are required for digital photography even if an optical viewfinder is not used. Using a camera’s live view feature or an electronic viewfinder is safe for the human eye, but the Sun’s rays could potentially irreparably damage digital image sensors unless the lens is covered by a properly designed solar filter.
Eclipse chasing
A dedicated group of eclipse chasers have pursued the observation of solar eclipses when they occur around the Earth. A person who chases eclipses is known as an umbraphile, meaning shadow lover. Umbraphiles travel for eclipses and use various tools to help view the sun including solar viewing glasses, also known as eclipse glasses, as well as telescopes.
Get in
Because solar eclipses, especially those viewable easily from land, are fairly rare, there are two problems with getting to them: the first is finding transport to the location at all and the second is booking it before your competition does. Solar eclipses can attract from thousands to hundreds of thousands of viewers, which can overwhelm the capacity of local transport and accommodation. If the eclipse is crossing a well-resourced tourist area (like the 2012 eclipse visible from Cairns, a tourist city in Australia), you should ideally book several months in advance but there may be some availability close to the eclipse. If the eclipse is off the beaten track, you may need to make arrangements a year or more in advance. Expect at least peak season pricing. Planes and trains will get booked up and tickets may become expensive, so book early and get seat reservations. If you waited too long and everything seems to be booked up, it might help to search for more creative options for transportation and accommodation—sometimes people will arrange charter buses or rent out their backyards to eclipse watchers. If you have the means, it’s wise to make bookings for two different locations; that way, a few days before the eclipse you can check the weather forecast and head to the spot with less chance of cloud cover.
Sometimes transportation companies offer special service with extra planes, trains, buses, or boats specifically for viewing the eclipse or getting people to good viewing locations. Cruise ships often have special itineraries into paths of totality, and this may be preferable if your willingness to travel off the beaten track is limited. They may also be able to search for a cloud-free viewing area within reasonable limits, an opportunity you are less likely to have on land. Helicopter or plane flights above cloud cover may be available if the eclipse is over an area with airstrips. While all of these tend to sell out rather quickly as well, they can be a unique way to make the journey relaxing and fun and to be among like-minded people on the way to the viewing destination.
Plan lots of extra time to get in and out. Especially in small towns that are good viewing locations, you can expect horrible traffic jams and crowded trains and buses. There are also likely to be more road accidents than usual caused by extra vehicles, tired drivers, and strained emergency services. In some cases, the journey can take 2–3 times what it normally would or even longer. Avoid the worst of the delays by arriving at your viewing location extra early and leaving late—ideally, arrive a day or two before the eclipse and depart a day or two after. Make contingency plans in case you end up being unable to get home when you thought you would. If driving, be prepared with different routes in case of traffic, and have the directions saved or printed out in case cell reception is poor.
Out-of-the-way places that are on the path of totality may see their populations swell by a factor of 10 or more for the eclipse. Local resources may be stretched beyond their limits, so bring extra food and water, and if you’re driving, make sure you have plenty of fuel. Be prepared for long lines to use toilets that may not be very clean. Don’t rely on your cell phone for communication or navigation—the local cell towers may be overwhelmed by the influx of people.
Bring an extra layer of clothing too—the decreased sunlight will make it chilly for a few minutes, even in a partial eclipse.
See
View the eclipse directly, by looking at the sun. See Stay safe: only totality can be observed safely without special precautions!
View the eclipse through a pinhole camera. Take two sheets of cardboard and make a small hole in one, and shine sunlight through that hole onto the other. The circle cast on the second sheet of cardboard (the equivalent of the retina of an eye or the sensor of a camera) will change its shape as a partial eclipse progresses.
View the eclipse from the ocean. In addition to cruise ship itineraries, coastal areas will often have day cruises available to view the eclipse, this may avoid crowded public areas, and the vessel may be able to avoid local cloud cover. As with transport in general, book early.
Photograph the eclipse. Beware: solar photography is not safe for camera sensors or film unless the lens is protected from the sun with a solar filter, which can be purchased from astronomy shops.
View the eclipse through a telescope. A solar filter must be over the lens of the telescope if viewing directly: some telescopes can be adapted to project an image instead.
Look at the strange crescent-shaped shadows during a partial, total, or annular eclipse. If you bring a straw hat with a loose weave, you can use it to produce dozens of the little crescents.
During a total eclipse, watch out for well-known phenomena before, during and after the eclipse:
Just before the eclipse, the enormous shadow of the moon will approach from the west at a very rapid pace: some people find this frightening. (If it’s a morning eclipse you may have your back to this phenomenon!)
Just prior to and after the eclipse, a phenomenon known as Baily’s beads may be visible, where the sun is first visible through craters on the side of the moon, creating points of light on the edge of the moon’s shadow.
During totality, birds may roost and generally the quiet of night will fall briefly. A partial eclipse will not visibly darken the surrounding countryside.
While ideally this is avoided by local authorities, it is not uncommon for automatic sensing streetlights to suddenly turn on, and rather spoil the show!
Learn
Eclipses provide a unique opportunity to do science. Famously, Einstein’s theory of relativity was tested during a 1919 eclipse over Príncipe which allowed researchers to see how the gravity of the sun caused starlight to bend. Nowadays, lots of science is done during eclipses, from observing the sun’s corona to investigating animal responses to the sudden change in light. Travellers are welcome and encouraged to participate, with both individual scientists and large organizations like NASA asking for amateur astronomers and ordinary people to help them gather data. Search for “eclipse citizen science” online to see how you can help.
Stay safe
Never look at the Sun with the unaided eye or with a camera or telescope, not even for a second and not even if only 1% of the Sun is visible. This may seriously damage your eye and even make you blind. Always use an approved solar filter either directly over your eyes for unaided viewing, or over the lens of a camera or telescope. You can use:
Eclipse glasses: CE certified or conforming to ISO 12312-2 or EN 1836 & AS/NZS 1338.1. These are usually cheap cardboard glasses costing around US$3 to $5.
Welder’s goggles rated 12–14, the highest ratings for blocking radiation.
Solar filters for cameras and telescopes, available from astronomy shops.
Eclipse glasses will often be available for sale at prime viewing locations. You might not need a pair for each person in your party—eclipses are slow enough that you should be able to hand a pair back and forth between two or three people if necessary.
If using a camera or telescope the lens itself must be protected: it is not sufficient to look through the viewfinder with eclipse glasses as the lens has magnified the sun’s power even further and may still damage your eye. In addition, the sun’s power will destroy camera sensors/film if you haven’t got a filter on the lens.
Do not use:
anything designed for vision/photography in normal bright light conditions, like sunglasses, or standard photography filters. These are millions of times less powerful than the filters you need to gaze at the sun.
lesser rated welder’s goggles
exposed film negatives
any stacked lesser protections
any non-certified protections
eclipse glasses with damage such as scratches or tears
Beware of fake eclipse glasses. Some unscrupulous manufacturers have placed the ISO logo on glasses that do not actually meet the organization’s standards, so make sure your glasses come from a reputable source. Glasses from science museums or astronomical organizations are almost certainly good to go; the American Astronomical Society also provides a partial list of reputable vendors.
As the moon fully obscures the sun during total eclipses it becomes safe to look without a filter and see the beautiful corona (the sun’s atmosphere). Have your eye protection ready for the end of totality.
Other observations
A total solar eclipse provides a rare opportunity to observe the corona (the outer layer of the Sun’s atmosphere). Normally this is not visible because the photosphere is much brighter than the corona. According to the point reached in the solar cycle, the corona may appear small and symmetric, or large and fuzzy. It is very hard to predict this in advance.
As the light filters through leaves of trees during a partial eclipse, the overlapping leaves create natural pinholes, displaying mini eclipses on the ground.
Phenomena associated with eclipses include shadow bands (also known as flying shadows), which are similar to shadows on the bottom of a swimming pool. They only occur just prior to and after totality, when a narrow solar crescent acts as an anisotropic light source.
Gravity anomalies
There is a long history of observations of gravity-related phenomena during solar eclipses, especially during the period of totality. In 1954, and again in 1959, Maurice Allais reported observations of strange and unexplained movement during solar eclipses. The reality of this phenomenon, named the Allais effect, has remained controversial. Similarly, in 1970, Saxl and Allen observed the sudden change in motion of a torsion pendulum; this phenomenon is called the Saxl effect.
Observation during the 1997 solar eclipse by Wang et al. suggested a possible gravitational shielding effect, which generated debate. In 2002, Wang and a collaborator published detailed data analysis, which suggested that the phenomenon still remains unexplained.
Eclipses and transits
In principle, the simultaneous occurrence of a Solar eclipse and a transit of a planet is possible. But these events are extremely rare because of their short durations. The next anticipated simultaneous occurrence of a Solar eclipse and a transit of Mercury will be on July 5, 6757, and a Solar eclipse and a transit of Venus is expected on April 5, 15232.
More common, but still infrequent, is a conjunction of a planet (especially, but not only, Mercury or Venus) at the time of a total solar eclipse, in which event the planet will be visible very near the eclipsed Sun, when without the eclipse it would have been lost in the Sun’s glare. At one time, some scientists hypothesized that there may be a planet (often given the name Vulcan) even closer to the Sun than Mercury; the only way to confirm its existence would have been to observe it in transit or during a total solar eclipse. No such planet was ever found, and general relativity has since explained the observations that led astronomers to suggest that Vulcan might exist.
Earthshine
During a total solar eclipse, the Moon’s shadow covers only a small fraction of the Earth. The Earth continues to receive at least 92 percent of the amount of sunlight it receives without an eclipse – more if the penumbra of the Moon’s shadow partly misses the Earth. Seen from the Moon, the Earth during a total solar eclipse is mostly brilliantly illuminated, with only a small dark patch showing the Moon’s shadow. The brilliantly-lit Earth reflects a lot of light to the Moon. If the corona of the eclipsed Sun were not present, the Moon, illuminated by earthlight, would be easily visible from Earth. This would be essentially the same as the earthshine which can frequently be seen when the Moon’s phase is a narrow crescent. In reality, the corona, though much less brilliant than the Sun’s photosphere, is much brighter than the Moon illuminated by earthlight. Therefore, by contrast, the Moon during a total solar eclipse appears to be black, with the corona surrounding it.
Artificial satellites
rtificial satellites can also pass in front of the Sun as seen from the Earth, but none is large enough to cause an eclipse. At the altitude of the International Space Station, for example, an object would need to be about 3.35 km (2.08 mi) across to blot the Sun out entirely. These transits are difficult to watch because the zone of visibility is very small. The satellite passes over the face of the Sun in about a second, typically. As with a transit of a planet, it will not get dark. The International Space Station transit across the Sun from any location can last from around 1 up to 8 seconds only taking into account, that the spacecraft is moving centrally alongside the diameter of the Sun. The longest International Space Station transits may occur just after the sunrise or just before the sunset when the way from observer to the object is the longest (see the Parallax phenomenon).
Observations of eclipses from spacecraft or artificial satellites orbiting above the Earth’s atmosphere are not subject to weather conditions. The crew of Gemini 12 observed a total solar eclipse from space in 1966. The partial phase of the 1999 total eclipse was visible from Mir.
During the Apollo–Soyuz Test Project conducted in July 1975, the Apollo spacecraft was positioned to create an artificial solar eclipse giving the Soyuz crew an opportunity to photograph the solar corona.
Impact
The solar eclipse of March 20, 2015, was the first occurrence of an eclipse estimated to potentially have a significant impact on the power system, with the electricity sector taking measures to mitigate any impact. The continental Europe and Great Britain synchronous areas were estimated to have about 90 gigawatts of solar power and it was estimated that production would temporarily decrease by up to 34 GW compared to a clear sky day. The temperature may decrease by 3 °C, and wind power potentially decreases as winds are reduced by 0.7 m/s.
In addition to the drop in light level and air temperature, animals change their behavior during totality. For example, birds and squirrels return to their nests and crickets chirp.
Recent and forthcoming solar eclipses
Eclipses only occur in the eclipse season, when the Sun is close to either the ascending or descending node of the Moon. Each eclipse is separated by one, five or six lunations (synodic months), and the midpoint of each season is separated by 173.3 days, which is the mean time for the Sun to travel from one node to the next. The period is a little less than half a calendar year because the lunar nodes slowly regress. Because 223 synodic months is roughly equal to 239 anomalistic months and 242 draconic months, eclipses with similar geometry recur 223 synodic months (about 6,585.3 days) apart. This period (18 years 11.3 days) is a saros. Because 223 synodic months is not identical to 239 anomalistic months or 242 draconic months, saros cycles do not endlessly repeat. Each cycle begins with the Moon’s shadow crossing the Earth near the north or south pole, and subsequent events progress toward the other pole until the Moon’s shadow misses the Earth and the series ends. Saros cycles are numbered; currently, cycles 117 to 156 are active.