by Joe Cali
A total solar eclipse occurs when the Moon passes between the Sun and the Earth. Total solar eclipses can only occur at some New Moon's but they don't occur every New Moon because the Moon's orbit is inclined 6o to the Earth's orbital plane. Eclipses only occur when the Moon passes through the Earth's orbital plane at New Moon otherwise the shadow misses the Earth (as illustrated in the diagram).
Even though the Moon crosses the Earth's orbital plane twice per month but there are only two 'eclipse seasons' per year when the Moon crosses at either new or full Moon when an eclipse is possible.
When do you need to protect your eyes?
There are three types of eclipse. Eye safety considerations are slightly different for each.
Type 1: Partial Eclipse.
On 8th April 2024, a partial eclipse will be observed outside the path of totality and for about 60-90 minutes before and after the total eclipse inside the path of totality. A partial eclipse is visible from a very wide zone many thousands of kilometres north and south of the centreline path of total or annular eclipse. The partial eclipse will be visible across the lower 48 states, all of Canada, Mexico, Guatemala, El Salvador, Nicaragua, Honduras, Costa Rica, Panama, all of the Caribbean, UK, Ireland, and the northwest tip of Spain.
During a partial eclipse, the lunar disk does not completely cover the Sun and so the bright surface of the Sun, the photosphere, is visible throughout the whole eclipse.
Primary Advice:
· There is no safe period when you do not need eye protection.
· Use eye protection for the whole eclipse!
Type 2 : Annular Eclipse.
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An annular eclipse occurs when the Moon is at the far part of its orbit at new moon. The lunar disk does not completely cover the Sun and so the bright surface of the Sun, the photosphere, is visible throughout the whole eclipse. No annular eclipse is visible on April 8th. The primary advice is the same as with a partial eclipse.
Primary Advice:
· There is no safe period when you do not need eye protection.
· Use eye protection for the whole eclipse!
Type 3 : TOTAL ECLIPSE
The Moon moves across the centre of the Sun and completely covers the Sun. A total eclipse begins and ends with a partial eclipse. During the brief period of totality, the Moon completely covers the Sun leaving no part of the bright disk, the Photosphere, visible. The brightest parts of the corona, prominences and the chromosphere, only visible through totality, are only about as bright as a full moon and present no risk of damage to the eyes. The diamond rings and Baily’s Beads can also be observed without eye protection.
Primary Advice:
· Use eye protection for the whole partial eclipse before and after the total eclipse.
· Eye protection is not required during totality. Use of a solar filter during totality will render the total eclipse invisible.
When is the safe period when you don't need eye protection?
The beginning and end of the safe period is marked by appearance of diamond rings. When this period starts, you will see the light on the landscape dim dramatically. The diamond rings marking the beginning and end of the safe period are very obvious and illustrated in the picture below.
The Sun goes through an 60-90 minutes of partial eclipse before and an hour after the total eclipse. During both periods, there is great danger of eye damage occurring if you don’t protect your eyes.
How to protect your eyes during the partial eclipse periods?
Whenever the bright disk of the sun is visible, you must use special techniques to protect your eyes. Looking at the bright disk of the Sun at any time during an eclipse or even on a normal day can burn the retina causing permanent eye damage. The Sun is no more dangerous during a partial eclipse than it is on any other day. On a normal day, if you glance at the Sun or get the Sun in your eyes for example while you are driving, you don't go blind. But you don't stare at the Sun. During the eclipse, you don't have to protect your eyes from glancing contact with the Sun any more than you do on a normal day. The problem is that during the eclipse, there is a temptation to stare at the eclipse for extended periods especially as the Sun gets covered. Staring can cause burns on the retina leading to permanent blind spots.
This is a picture of the retina belonging to Ian Poole of Canberra, Australia. As a child, Ian watched the partial phase before the 1976 total eclipse without eye protection from the back seat of his parents car and received this retinal burn. His parents were not aware of what he was doing and they had warned him. He suffered a resulting lifelong blind spot. Such injuries are uncommon and easily prevented by taking the simple precautions described.
Using found materials from around the home as de facto solar filters is very dangerous. These materials might resemble solar filters but they don't filter the dangerous UV and IR rays and can lead to blind spots even though the image looks dark enough and comfortable to your eye. Examples of unsuitable filters include pinholes, photographic film negatives, cd's/DVD's, smoked glass and silvered confectionery wrappers among many others. There are no materials found around the home that will safely filter the Sun's high intensity light. Some of these materials might look similar to solar filters but they are definitely not safe. The best policy is to assume that if it hasn't been made for the purpose, it isn't safe.
Even during the final minute when the Sun is almost completely covered and the ambient lighting begins to look like twilight, the energy in the very thin crescent is more than capable of causing a permanent burn to your retina.
There are two accepted safe methods for watching a partial eclipse. One is to look through specially made filter material, the other is to use some sort of projection method.
Method #1 Filters
Eclipse shades, also called eclipse glasses are special cardboard frame sunglasses with properly made solar filters built in. These are the most convenient filtration method. Do read the instructions. Some of these shades are not rated for continuous viewing but for viewing 30 seconds at a time. These cardboard glasses use special filter materials that effectively cut all wavelengths of light including the dangerous IR and UV that de facto materials pass to your eyes. Your eyesight is worth protecting. Other suitable filters are a number 14 arc welding filter available from welding suppliers or a special solar filters available from astronomical suppliers designed to be attached to telescopes or binoculars.
Check that any eclipse glasses or solar filters are marked with a statement that they are : -
“certified to comply with the requirements of the ISO 12312-2 international standard.”
ISO 12312-2 is a standard that has been developed by collaboration between solar eclipse observers and health authorities to ensure safe blockage of the visible wavelengths and the damaging yet invisible UV and IR wavelengths.
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Girl in a bus, Jinta, Gansu Province, China testing her eclipse shades before the commencement of the 2008 total eclipse.
Method #2 Pinhole or mirror projection
The oldest safe method is the projection method. Cut the side out of a large cardboard box. Punch a 5mm hole in the side.
If the Sun will be low in the sky as the eclipse begins. Stand about 3-5m away from the wall of a white walled building. With your back to the Sun hold the card up so that its shadow falls on the wall. In the middle you will see a small image of the partially eclipsed Sun. If the sun is high in the sky, you'll need to find a higher location and project downwards a few metres onto a piece of white card or cloth on the ground.
A mirror pinhole can be much more convenient. A small flat mirror covered by a piece of paper with a 5mm hole projected onto a shaded flat white surface such as the side of a car or a white wall on a building will produce an image of the partially eclipsed sun.
The example shown is not great, the wall of the caravan was corrugated reflective metal yet it still showing the partial phase. The hole over the mirror was about 5mm and projection distance was about 20 metres. The projected image was about 180mm diameter. The blur on the edges is about 2 x the hole diameter, 10mm all around the edges.
Where to go?
A partial eclipse is visible from all countries in Central and North America including 49 states of the USA. Only Alaska misses out. So, if your aspiration is to see a partial eclipse, stay home. I have sometimes heard people say, oh well we saw a 99% partial eclipse, that’s only 1% off a total eclipse. Experienced eclipse chasers will tell you that a 99% partial eclipse is 100% off a total eclipse. If you live near the path of totality, try to get there. This is the most accessible eclipse for many years. There is another total eclipse that skims the north coast of Alaska in 2033, another in Canada in 2044 but the next C48 eclipse is in 2045, 21 years from now.
If you do travel, be prepared, and leave early or have accommodation pre-booked. Heavy traffic may limit road speed to less than 20 miles per hour. The map on the next page shows the path of totality and point of greatest eclipse in Mexico. The large area covering most of North and Central America, Caribbean and the north Atlantic shaded grey shows the massive area where a partial eclipse can be observed.
For detailed geographic information down to USA state level path maps, there is no better resource than this page created by my friend Michael Zeiler, https://www.greatamericaneclipse.com/april-8-2024. Hard top accommodation within the total eclipse path has been quite sparse and highly inflated in price for a couple of years. There is plenty of campsites and RV parking being made available on private farms and properties.
Solar filters
In addition to eclipse glasses for safe naked eye viewing, you might want to have filters for a camera lens, binoculars or a telescope. There are many commercial solutions available. Always make sure you buy filters that are certified to comply with the requirements of the ISO 12312-2 international standard.
Mounting filters
I have been chasing eclipses for 30 years since the annular and total eclipses of 1994. My equipment has changed many times. As totality is about to begin, it is important that you can remove the solar filter easily, quickly, and applying minimal force to the scope or camera lens so that you do not shift the pointing or on a camera lens, shift the focus. At the same time, the mounting must be very secure so that it cannot casually blow off in a stiff breeze. Mountings must also be light tight. For the past 20 years, I have used a home-made mounting system for certified solar film. The filter is held in one board that clips to a flange board attaches around the scope dew hood. I also have a universal screw mounting flange for camera lenses and a friction mounting flange for telescope lens hoods.
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I have a detailed description how to make both types of filter on my website.
https://joe-cali.com/eclipses/EQUIPMENT ... lters.html
IMPORTANT: You must use certified filter material.
Manufacturers / Sources of solar filters and eclipse glasses
Eclipse Glasses
Rainbow Symphony
http://www.rainbowsymphony.net/eclipse-glasses.html
Seymour Solar
https://www.seymoursolar.com/
Solar Filters (sheet film or glass)
Baader Planetarium - Astrosolar safety film
http://protecsolar.com/en/
Thousand Oaks Optical
http://www.thousandoaksoptical.com/
The links above are the manufacturers. Many of these products are sold by most reputable astronomical product retailers
Links
"Eclipse eye safety" by Ralph Chou on NASA eclipse home page
http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html from the NASA eclipse home page
"OK, Look directly at a Total Solar Eclipse" by Dick Land, The Schepens Eye Research Institute
http://web.williams.edu/Astronomy/IAU_e ... lipse.html
Bill Kramer's Eclipse Chasers web site has an Eye Safety page http://eclipse-chasers.com/tseSAFE.html
Astronomical Association of Queensland - Eclipse Eye safety https://eclipse.aaq.org.
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Joe Cali is an Australian astronomer, astrophotographer, and experienced eclipse chaser & photographer. Joe has chased fifteen total and 3 annular eclipses around the world over the past 30 years. His eclipse work has been published in high circulation magazines Australian Geographic, Sky and Telescope, and Australian Sky and Telescope, and extensively on his web site.
A large section of his personal website is devoted to solar eclipse chasing. https://joe-cali.com/eclipses/
