Dominic Ford, Editor
From the Eclipses feed
The Moon will pass in front of the Sun, creating a total eclipse of the Sun visible from United States between 11:20 and 17:18 EDT.
The path of the total eclipse will pass through United States (change location ).
The map below shows the parts of the world where the eclipse will be visible, which are highlighted within the red contour. The yellow contours show the maximum extent of the eclipse, where the Moon appears to cover 20%, 40%, 60% and 80% of the Sun.
The eclipse geometry
Solar eclipses occur when the Sun, Moon and Earth are aligned in an almost exact straight line, with the Moon in the middle, such that the Moon passes in front of the Sun. The diagram to the right shows this geometry, though for clarity the Moon is drawn much closer to the Earth than it really is.
The Moon passes close to the Sun in the sky every month, at new moon, but because the Moon's orbit around the Earth is tipped up by 5° relative to the Earth's orbit around the Sun, the alignment usually isn't exact.
In the diagram below, the grid represents the plane of the Earth's orbit around the Sun. As it circles the Earth, the Moon passes through the Earth–Sun plane twice each month, at the points on the left and right labelled as nodes. A solar eclipse results when one of these node crossings happens to coincide with new moon, which happens roughly once every six months. At other times, the Moon typically passes a few degrees to the side of the Sun at new moon.
Even when a solar eclipse does occur, it will not be visible from the whole world.
The Moon is much smaller than the Earth, and so the shadow that it casts onto the Earth is never more than a few hundred miles across. As the Moon moves relative to us, the shadow sweeps across the Earth, so that different places see the eclipse at different times.
The diagram below shows the Moon's shadow, with the Earth, Moon, and distance between them, drawn precisely to scale. The pink region shows the region of space where the Moon would appear to completely cover the Sun, creating a total solar eclipse. The blue region shows where the Moon would appear to partially cover the Sun, creating a partial solar eclipse.
The Earth is drawn twice on the right hand side, once at its closest possible distance from the Moon (left), and then again at its furthest possible distance from the Moon (right).
The cross marks the maximum distance from Moon at which a total eclipse is possible. Beyond this, the Moon appears too small to entirely cover the Sun.
For comparison, the geometry of lunar eclipses is also shown below: the Earth's shadow is by contrast to the Moon's shadow, amply large enough to cover the whole Moon at once, as happens in a total lunar eclipse.
The eclipse path
The simulation to the right shows the path of the Moon's shadow across the Earth.
The red line shows the edge of the Moon's shadow: all places inside the red circle will see the Moon covering some part of the Sun's disk. The yellow contours within this show where the Moon appears to cover 20%, 40%, 60% and 80% of the Sun.
The yellow spot in the centre of the Moon's shadow traces out the thin eclipse track where a total eclipse will be seen.
The total eclipse will be visible from United States.
Additionally, a partial eclipse will be much more widely visible, from countries including:
|Antigua and Barbuda||78%|
|British Virgin Islands||76%|
From Ashburn (select a different location), the percentage of the Sun's disk covered by the Moon will proceed as follows:
As seen from any given location, this total eclipse will last for a maximum of 2 minutes.
Below, the path of the Moon's shadow is projected onto a flat map of the world. As above, the red contour shows the edge of the Moon's shadow, and encloses everywhere where the eclipse can be seen. The yellow contours show where the Sun is 20%, 40%, 60% and 80% covered.
Observing the Sun can be very dangerous if it is not done with the right equipment. The Sun is the brightest object in the sky, and looking directly at it can cause permanent eye damage within seconds. Viewing it through any optical instrument – even a pair of binoculars or the finderscope on the side of your telescope – can cause instant and permanent blindness.
If you have any doubts about whether your equipment is safe, it is best not to risk using it. By far the safest thing to do is to go along to a public observing event. Many astronomical societies are likely to be hosting observing events on the day, and they'll be sure to welcome newcomers. You may meet some new people at the same time as seeing the transit.
Many astronomy suppliers sell special special filters which are made for safe solar viewing. These include aluminised mylar filters, or black polymer filters, identified as suitable for direct viewing of the Sun. Check that the filter has a CE mark, and a statement that it conforms to European Community Directive 89/686/EEC. Alternatively, you can use a welder's glass rated at No. 14 or higher. Always read the manufacturer's instructions carefully.
Never attempt to make your own filter. In addition to visible light, the Sun also produces prodigious amounts of infrared and ultraviolet radiation which cannot be seen yet can still damage your eye. Even if a homebrew filter appears adequate, it may allow this unseen radiation to pass.
Projecting an image of the Sun
Another safe way to view solar eclipses is to buy a purpose-built solar projection box.
These typically consist of a cardboard box with a small lens on one side. They project an enlarged image of the Sun onto a white cardboard sheet inside the box. Once the transit is over, they're also great for observing sunspots. They are safe to use, quick to set up, and ideal for use with children and groups.
This eclipse is a member of Saros series 145. The position of the Sun at the moment of greatest eclipse will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 21 August 2017|
29 days old
All times shown in EDT.
Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.
The simulations above were derived from the DE405 ephemeris published by the Jet Propulsion Laboratory (JPL). The position of the Moon's shadow is superimposed on maps of the world taken from the NASA Visible Earth project.
The list of countries from which the eclipse is visible was computed on the basis of shape files available from DIVA-GIS.
Additional information was taken from:
Espanak, F., & Meeus, J., Five Millennium Canon of Solar Eclipses: -1999 to +3000, NASA Technical Publication TP-2006-214141 (2006)
You may embed the animations and images above in your own website. They are licensed under the Creative Commons Attribution 3.0 Unported license, which allows you to copy and/or modify them, so long as you credit In-The-Sky.org.
|14 Aug 2017, 21:16 EDT||– Moon at Last Quarter|
|21 Aug 2017, 14:31 EDT||– New Moon|
|29 Aug 2017, 04:14 EDT||– Moon at First Quarter|
|06 Sep 2017, 03:04 EDT||– Full Moon|
© Lutfar Rahman Nirjhar. Image taken in July 2009.