What is a solar eclipse?
Our planet Earth has a natural satellite, the Moon. It takes less than one month to the Moon to complete an orbit around us. Of course the Earth and the Moon, as a couple, are orbiting around the Sun in one year.
Because of its motion around us, we see the Moon moving in the sky, mainly across the Zodiacal constellations. But we are orbiting around the Sun, so from the Earth we also see the Sun apparently moving along the ecliptic, the projection in the sky of our orbit around that star.
In short, even if for different reasons, both the Sun and the Moon are moving in our sky. The plane containing the orbit of the Moon has an inclination of almost 6 degrees respect to the ecliptic plane, otherwise the Moon, too, would move on the same apparent celestial path of the Sun, the ecliptic.
While orbiting around us, at some point the Moon will show in the direction of the Sun (new Moon) or to its opposite (full Moon). If both the Sun and the Moon would move along the ecliptic, we would have a solar eclipse at every new Moon (being our satellite in front of the Sun: moving on the same path!) and a lunar eclipse at every full Moon (the Moon would be inside the Earth’s shadow, cast on the ecliptic plane!).
Because of those almost 6 degrees between the two planes, having an eclipse is not frequent, as the alignment mentioned above is not obvious. Here it is a side view of this.
Of course, while orbiting around us, the Moon can be above or below the apparent celestial path of the Sun (we repeat, its apparent path is called ecliptic). When it moves from above to below and vice versa, the Moon as to cross the ecliptic: those two intersection points are named “nodes”.
Those “nodes” are essential for the eclipses. When the Moon is on the nodes, it is on the ecliptic. The question is: where is the Sun in that moment?
If in that very moment the Sun is in the same node, too, then we will have a solar eclipse (and of course, the new Moon), while if the Sun is on the other node, our star, our planet and the Moon will be aligned and the shadow of the Earth will cast on the Moon: here it comes a lunar eclipse (and the full Moon).
Of course, the alignment required for the eclipses (both solar and lunar ones) can be more or less perfect, so the eclipses can be partial or total: for the solar ones, it will be partial when the Moon will cover only part of the Sun, total when the Sun will be completely covered by our satellite. Furthermore, because the lunar orbit is elliptic, its distance from us varies (from about 360.000 km – 225.000 miles to about 405.000 km – 251.000 miles) and, as a consequence, the apparent angular size of the lunar disk varies, too. This affects the solar eclipses, no matter the quality of the alignment mentioned before.
Because the Sun and the Moon disks have almost exactly the same angular size (about half-a-degree), those variations matter: if when both the Sun and the Moon are in the same node, our satellite is too far, the lunar disk will be smaller than the one of the Sun, resulting in an “annular” eclipse.
Even when the Moon is at its minimum distance from us, the angular size of its disk is just a bit larger than the one of the Sun, so even in this ideal case the totality will last a few minutes (the maximum duration possible, considering the factors above and a few more, subtle ones, is a bit more than 7 minutes).
Another consequence of the marginal difference in angular size between the Sun and the Moon is that even the total solar eclipses will be total only from a very narrow strip on Earth. If you are inside it, you will see the total event (the closer to the center of that path the better) and there will be a place along that path from where you will have the greatest eclipse and greatest duration events: for total solar eclipses they are coincident within a fraction of second.
The 21 Aug. 2017 solar eclipse will be total, with a maximum duration of the totality of 2 minutes and 40 seconds in Kentucky.
NOTE: observing the Sun without the right, safe protections can seriously and permanently damage your eyes. Be extremely careful and be sure to follow the instructions from reliable sources, as this site. DURING TOTALITY ONLY YOU CAN OBSERVE WITHOUT FILTERS!
How to observe the solar eclipse.
The observation of a solar eclipse has only one very important, mandatory obligation: safety! The Sun is so bright that even a very quick sight, with naked eyes, can result in damaging them, even permanent. There is no eclipse more precious of your own eyes, so be always very careful when observing the solar eclipse. If you have a doubt about the safety of what you are doing, please stop: you can have other eclipses in the future, but no new eyes.
NEVER LOOK AT THE SUN WITHOUT THE PROPER, CERTIFIED TOOLS: NEVER.
The confidence we have with our star can persuade us to look at it without protection, but this MUST be ABSOLUTELY AVOIDED. You can observe with your own eyes ONLY DURING TOTALITY.
That said, enjoying the unique show of a solar eclipse in full safety is very simple, as you must follow some easy procedures and/or use very simple tools.
Here we will share a few suggestions and ways to look at the Sun during the eclipse, with the hope to help you to experience one of the most beautiful natural events and exploit it as much as possible.
- Pinhole devices. Remembering the dawn of photography, you can easily make a simple darkroom, doing a very small hole with a pin or needle on a large enough piece of cardboard, using it as an optics to project on a rear screen, placed at about one meter from the hole. Making an hole of the right size is important: too large holes would provide blur images, too small ones would give too faint images. The best option would be to create enough shadow all around the hole, making the hole on a wide cardboard and using its own shadow. Also, some kind of box as below can be useful.
For the best results, it is better to experiment in due time with different size holes and different distances of the rear screen.
Of course, this device helps during the partial phase of the eclipse. During totality ONLY, you can observe by naked eye and enjoy the stunning solar corona.
NOTE: DURING TOTALITY ONLY YOU CAN OBSERVE WITHOUT FILTERS!
- Projection with an optical device. A small telescope or binocular can be used to project (without looking through it!) the image of the Sun on a screen. You must be very careful not to cross the light beam coming for the instrument: in particular, you have to be very careful when pointing it to the Sun, as you never have to look directly at our star, never. You can easily point the Sun looking at the shadow of your device on the screen. If you are using a binocular, keep the cap on one of the two lenses, to avoid a double image.
If you are using a telescope, keep the cap on the finderscope, to avoid burning its crosshair (NEVER use it to point the scope to the Sun: again , use the shadow of the tube, it is easy!); this way, you will also be sure you will never look through it by accident.
It is worth to note that a telescope used to project the Sun this way concentrates a lot of heat on the lenses of the eyepiece, with the risk to have it damaged, is its lenses are stuck together. Even for this (and to avoid to stress your optics with these thermal shocks (to reduce them you can diaphragm the lens, a concept well known in photography), the following option, which uses a special film, is by far the best solution for a memorable and safe experience.
NOTE: DURING TOTALITY ONLY YOU CAN OBSERVE WITHOUT FILTERS!
- AstroSolar film. The techniques mentioned above suggested to observe the solar eclipse in an indirect way, projecting the image of the Sun on a screen. This is because the instruments used there were not equipped with the indispensable devices needed to safely observe our star directly. But, of course, a telescope properly equipped with the right tools to directly observe the Sun will offer a much better experience. And to make your scope ready to safely do this is incredibly simple and cheap.
All you need is a special film called Astrosolar, very useful to create custom-made filters for an absolutely safe observing experience, very useful both visually (even without a telescope!) and for imaging purposes.
Using the film above, you can cut it to fit the lenses of your observing device.
The simplest, completely safe way to observe the solar eclipse (and the Sun in general) is with a pair of special solar glasses made with Astrosolar, ready to wear and use. Considering how cheap, how simple to use and, above all, how completely safe they are , these special glasses can be considered a must for solar and eclipse observing. It is worth to underline that these glasses can be very useful to look at the Sun when big sunspots are showing, as in these days, visible with (protected!) naked eyes.
NOTE: DURING TOTALITY ONLY YOU CAN OBSERVE WITHOUT FILTERS: TAKE THEM OFF.
- Special, solar telescopes. During the partiality, you can follow the ongoing eclipse with special solar telescope, for example H-alpha ones, making the vision mind-blowing. Thjese telescopes will add the visibility of amazing solar features as prominences and the typical structure of the chromosphere.
DURING TOTALITY ONLY YOU HAVE TO OBSERVE WITHOUT H-ALPHA FILTERS