The Total Solar Eclipse of 11 July 1991. The dark blue region is the path of totality (also known as the umbra).
The umbra shadow travels from west to east (left to right on the map). At the point of greatest eclipse, totality is at local midday. The duration of totality at greatest eclipse is 6 minutes 58 seconds. This occurs in Mexico. On either side of that point, the duration is less. To the West of greatest eclipse the eclipse will be total before local noon; to the east, the eclipse will be total after local noon. The umbra is widest around the time of greatest eclipse as the surface of the Earth is bulging out towards the Moon in that region.
The blue circlular regions are the positions of the umbra at ten minute intervals. The umbra becomes more circular around the region of greatest eclipse as the meeting between the Earth and the umbra becomes more perpendicular. The sub-solar point is the location where the Sun is overhead at the time of greatest eclipse. For this eclipse the sub-solar point and location of greatest eclipse are almost identical. This is because the Sun is almost overhead at the time of greatest eclipse. The alignment of Earth, Moon and Sun is very exact for this eclipse.
The red regions are the areas where the eclipse occurs during sunrise (left) or sunset (right). The area on either side of the path of totality (in pale blue) provides a partial eclipse, the magnitude decreasing with distance from the path of totality. Beyond the blue lines, no eclipse is visible.
The path of totality begins in the Pacific Ocean, crossing the islands of Hawaii. It then passes into Mexico and travels through Central America, Colombia and Brazil from where it leaves the Earth. The umbra takes 3 hours 23 minutes to traverse its entire path. Unlike most eclipses which cover uninhabited regions of the Earth, this eclipse was total in five capital cities: Mexico City, Guatemala City, San Salvador, Tegucigalpa and Mangua.
Our eclipse site was close to the southern tip of Baja California, a peninsula that is part of Mexico, close to the point of greatest eclipse. This was the longest eclipse I would ever see as no longer one occurs for over 100 years.
| Location | near Santiago, Baja California, Mexico |
| Distance from Centre Line | 0 km |
| 1st Contact (UT - 7) | 10:24 |
| 2nd Contact | 11:49 |
| 3rd Contact | 11:56 |
| 4th Contact | |
| Duration | 6m 58s |
| Path Width | 258 km |
| Umbral Velocity | 0.55 km/s |
| Solar Altitude | 88° |
| Solar Azimuth | 304° |
| Saros Details | 136 (36 / 71) |
| Gamma | -0.0044 |
| Diameter Ratio | 1.080 |
The Distance from Centre Line was estimated from GPS and a good quality map plotted with the path of the centre line using data from Fred Espenak.
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; we had left the site by then. The times are in local time which, for this eclipse, is UT - 7 (GMT minus seven hours).
The Duration of the eclipse at the observation site was 6 minutes 58 seconds. This is the longest duration for the next 140 years.
The Path Width is the width of the path of totality. The value of 258km is a large figure that produced a long eclipse.
The Umbral Velocity is the speed of the Moon's shadow, 500 metres per second. This relatively slow speed coupled with the large path width produced a very long 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 Sun was virtually overhead for this eclipse.
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, 11 days and 8 hours later. This eclipse is a member of Saros number 136. It is the 36th eclipse out of a total of 71 in the series. The three previous eclipses of this series occured in 1973, 1955 and 1937. All three were over 7 minutes long, the longest eclipses of the 20th century.
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 -0.0044. This means that the shadow passes a very small amount South of the Earth's centre; the alignment between the Earth, Moon and Sun is almost exact. In July, the Northern Hemisphere is tilted towards the Sun. This produces an eclipse in the Northern Tropical Zone with the Sun almost overhead at the point of greatest eclipse.
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 1.080 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. This value is almost as large as it is possible to have. In early July the Sun is at its furthest from the Earth, hence appearing smaller in the sky. In addition, the Moon was very close to the Earth in its monthly orbit, hence appearing larger in the sky. These combinations conspired to produce a very long eclipse, only 34 seconds shorter than the theoretical maximum. The Moon was 8% larger than the Sun. During the eclipse, the movement of the Moon could be seen as solar atmospheric features could be seen, first on one side of the Sun, then on the other.
