The Total Solar Eclipse of 21 August 2017.
The narrow blue region is the Path of Totality (also known as the Umbra).
The Moon's umbra (shadow) travels from west to east (left to right and bottom to top on the map). At the point of Greatest Eclipse, totality is at local midday. The duration of totality at greatest eclipse is 2 minutes 40 seconds. This occurs close to the town of Carbondale, Illinois. On either side of that point, the duration of totality is less. To the West (left and lower) of the point of greatest eclipse, the eclipse is total before local noon; East (right and upper) of this point, totality is seen after local noon. The umbra is generally widest around the time of greatest eclipse as the Earth is bulging out towards the Moon in that region. This brings the Earth's surface closer to the Moon.
The area on either side of the path of totality (in pale blue) is called the Penumbra and provides a partial eclipse, the magnitude decreasing with distance from the path of totality. Beyond the partial region no eclipse is visible.
The Sub-solar Point is the location where the Sun is overhead at the time of greatest eclipse; in August this point is North of the equator. The parts in red on either end of the umbra are the regions where the eclipse occurs during sunrise (left) or sunset (right).
The Moon's shadow (Umbra) touched down on the Earth at 16:48 (UT) in the Pacific Ocean South of Alaska. At this point the width of the shadow was 62km and the duration of totality was 0m 52s at local sunrise.
The shadow then moveed Eastwards across the sea for 28 minutes travelling at just under 7km per second when it first touched Earth and slowing down as the Earth bulged up to meet it. It reached dry land in the North Western USA at 17:16 when it was travelling at 1.2km per second. On the coast the path width was 99km and the duration of totality was 1m 59s. Here the eclipse was total at mid-morning.
The shadow then crossed several states of the USA including Oregon, Idaho and Wyoming.
The umbra continued through Nebraska, the Kansas - Missouri border and into a small part of Illinois.
The small town of Carbondale is close to the place of maximum duration (2m 40s) and the widest shadow path (115km) moving at 650 m per second. The local time of totality was around 1:20pm in this region.
Totality is longest here because this is the part of the Earth closest to the moon during the eclipse. The umbra was its widest and is moving at its slowest - this gives more shadow taking longer to pass.
Carbondale has a second claim to fame. It is also within the path of totality of another total eclipse in 2024. This small town will see two total eclipses of the Sun in seven years. The average wait for two total eclipses of the Sun in one place is about 400 years.
The umbra then continued through Kentucky, Tennessee, Georgia and South Carolina. It left North America at 18:48 travelling at 670 m per second with a path width of 114km and a duration of 2m 34s. The eclipse was now occurring in the afternoon (local time).
The umbra spent the next 75 minutes moving over the Atlantic Ocean. There were no major landfalls during this period.
The umbra finally left the Earth at 20:02 (UT) in the ocean south of the Cape Verde Islands. The duration of totality was now 0m 47s and the path width had shrunk to 57km. The shadow swept off the Earth travelling at more than 4km per second.
The umbra took 3 hours 13 minutes to traverse its entire path, marking out a narrow band with a length of 13,900km and a maximum width of 115km. Totality will have been visible over an area covering 0.26% of the Earth's surface.
The Path of Totality (the Moon's Umbra) is the shaded area moving across the USA from top left to lower right.
The Moon's shadow took about 90 minutes to transverse the USA.
The green flag marks the region where the duration of totality is at its longest (2m 40s).
Yellowstone National Park was outside the path of totality. The city of Jackson in the Grand Tetons National Park was close to the centre line and experienced totality of 2m 18s.
Casper, even closer to the centre line, had totality of 2m 26s.
At Douglas totality was 2m 20s.
The Path of Totality and the elliptical shape of the Umbra over Casper.
The Moon's Umbra was falling onto the Earth at a slight angle. This produced the elliptical shape. The shadow was 123km long (the Major Axis) by 99km wide (the Minor Axis).
The Path Width was 108.4km (67.4 miles).
We were at Fields Creek, on the Coates Road south of the centre line (marked with a yellow x).
Venus was the brightest planet visible during totality and was spotted shortly before totality to the right of the Sun,
During totality the stars Sirius and Regulus were visible, the latter two moon-widths to the left of the Sun.
| Date | 21 August 2017 |
| Location | Fields Creek, Casper, Wyoming, USA |
| Latitude | 42° 46' 41" N |
| Longitude | 106° 23' 37" W |
| Altitude | 1570 m |
| Distance from Centre Line | 6.2 km (3.8 miles) |
| 1st Contact (UT - 6) | 10:22 |
| 2nd Contact | 11:42:30 |
| 3rd Contact | 11:44:58 |
| 4th Contact | 13:09 |
| Duration of Totality | 2m 28s |
| Path Width | 108.4 km (67.4 miles) |
| Major Axis | 123 km |
| Minor Axis | 99 km |
| Direction of Shadow Approach | ~ 285° |
| Umbral Velocity | 0.767 km/s (1715 mph) |
| Solar Altitude | 54° |
| Solar Azimuth | 143° |
| Position Angle: 2nd Contact | 104° |
| Position Angle: 3rd Contact | 294° |
| Gamma | 0.4369 |
| Diameter Ratio | 1.029 |
| Magnitude | 1.013 |
| Saros Details | 145 (22 / 77) |
The Location, Latitude, Longitude and Altitude (in metres) are for the observation site at Fields Creek just South West of Casper.
The Distance from Centre Line was estimated from Google Interactive maps provided by Fred Espenak and Xavier Jubier.
First Contact is the beginning of the eclipse when the first "bite" appears on the Sun's disk; it is the beginning of the partial phase. Second Contact is the beginning of totality. Third Contact is the end of totality. Fourth Contact is the end of the partial eclipse. The times are in local time which, for this eclipse, is UT (GMT) - 6.
The Duration of the eclipse at the observation site was 2 minutes 28 seconds. This was 2s longer than the duration at the centre line because of the exact shape of the Moon.
The Path Width is the width of the path of totality. The umbra itself is slightly elliptical in shape. The Major Axis is the longest axis of the umbra; the Minor Axis is the shorter axis. The umbra at the observation site was 123 km long and 99 km wide and had a path width of 108.4km (67.4 miles).
The Direction of the Shadow indicates that the umbra approached the observation site from North of West.
The Umbral Velocity is the speed of the Moon's shadow (just under 767 metres per second, 1715 miles per hour) at the observation site. This, and the size and shape of the umbra, determines the duration of the total eclipse.
The Sun's Altitude is measured from the horizon; the Azimuth is the direction of the Sun measured clockwise from North. The figures are for mid-eclipse. The figures mean that the Sun was more than half way up over the South Eastern Horizon.
The Position Angles indicate the exact position of the Sun's disk where the Moon covers and uncovers the Sun at the beginning and end of Totality. These figures determine the position of any diamond rings or Bailey's beads at the beginning and end of totality.
Gamma determines how the Moon's shadow, if extended, would pass through the Earth.
A Gamma of zero implies that the shadow would pass through the exact centre of the Earth. A Gamma of greater than 1 misses the Earth and no total eclipse would occur. A positive Gamma passes North of the Earth's centre; a negative Gamma passes South of the Earth's centre. The value of Gamma for this eclipse is 0.4369. This means that the shadow passes just under half of the way between the Earth's centre and the Northern part of the Earth. This, combined with the fact that in August, the North Pole is tilted towards the Sun, produces an eclipse mainly in the Northern Hemisphere.
The Diameter Ratio determines how much bigger the Moon's apparent radius is than the Sun's. In this case, the Moon's radius is 1.029 that of the Sun's. A total eclipse can only occur if this figure is greater than 1. The Moon would then appear larger than the Sun and could cover it completely. If this figure was less than one a total eclipse could not occur because the Moon would appear smaller than the Sun.
For this eclipse the value of the Diameter Ratio means that the Moon appears just under 3% larger than the Sun.
The Magnitude determines how much larger the Moon's apparent area is than the Sun's. For this eclipse the magnitude is 1.013.
The Saros is a collection of eclipses belonging to a series. Each member of the series is followed by a similar eclipse approximately 18 years, 10 / 11 days and 8 hours later.
The eight hours has an interesting effect. The 1999 eclipse was total in Europe and Asia. The eight hours between the 1999 and the 2017 eclipses means that the Earth has rotated an extra third of the way round. Eight hours is a third of a day. So the 2017 eclipse is a third of the way round from Europe and Asia and is occurring over North America instead. The next member of this series will be on 2 September 2035 and that third of a rotation will see it being total in East Asia and the Pacific. Each successive eclipse in a Saros is about 120 degrees West of its predecessor.
This eclipse is a member of Saros number 145. It is the 22nd eclipse out of a total of 77 in the series.
The series began on 4 January 1639 with a small partial eclipse (less than 1%) in the Arctic regions. The Moon's umbra was a long way from the Earth and only a small part of the penumbra brushed the Earth.
This was followed by 13 partial eclipses of increasing magnitude as the Moon's shadow moved closer to the Earth's centre with each successive eclipse.
On 6 June 1891, the Moon's shadow finally made contact with the Earth. However, the Moon was too far away to cover the Sun completely and a ring-shaped partial eclipse occurred. This type of eclipse is called an annular (from the Latin for ring).
For the next eclipse (17 June 1909), the Moon was a bit closer to the Earth. This eclipse was a hybrid. Along some of its route the eclipse was annular but in those parts where the Earth was closer to the Moon a total eclipse occurred. The maximum duration of totality was just 24s as the Moon was only just big enough to cover the Sun.
The next eclipse of the series (29 June 1927) was the first wholly total eclipse of the series. It was one of only two total eclipse visible over England in the 20th century. The eclipse was always less than 50s long.
The next few eclipses of the series were all totals, moving Southwards and of increasing duration.
On 11 August 1999, the previous eclipse of Saros 145 occurred. This was the second and final eclipse total over England in the 20th century. Kryss and Talaat were in Cornwall where the weather was sunny until eclipse day.
The duration of these eclipses will continue increasing and the path of totality will continue to move southwards. On 25 June 2522 an eclipse with duration 7m 12s will occur. Total eclipses over 7 minutes are very rare.
After seven more totals (the final one on 9 September 2648), a series of 20 decreasing partial eclipses in the Antarctic will see the series end with a 6% partial on 17 April 3009.
Saros 145 will consist of 77 eclipses: 34 partials, 1 annular, 1 hybrid and 41 totals. The series lasts for 1370 years. At any one time dozens of Saros series are in progress. Other eclipses will belong to different series.
