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An annular solar eclipse occurred at the Moon's descending node of orbit on Monday, April 18, 1977,[1] with a magnitude of 0.9449. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. An annular solar eclipse occurs when the Moon's apparent diameter is smaller than the Sun's, blocking most of the Sun's light and causing the Sun to look like an annulus (ring). An annular eclipse appears as a partial eclipse over a region of the Earth thousands of kilometres wide. Occurring about 3.1 days before apogee (on April 21, 1977, at 13:00 UTC), the Moon's apparent diameter was smaller.[2]
Annularity was visible in South West Africa (today's Namibia), Angola, Zambia, southeastern Zaire (today's Democratic Republic of Congo), northern Malawi, Tanzania, Seychelles and the whole British Indian Ocean Territory. A partial eclipse was visible for parts of eastern Brazil, Southern Africa, Central Africa, East Africa, Antarctica, the Middle East, and South Asia.
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Shown below are two tables displaying details about this particular solar eclipse. The first table outlines times at which the moon's penumbra or umbra attains the specific parameter, and the second table describes various other parameters pertaining to this eclipse.[3]
| Event | Time (UTC) |
|---|---|
| First Penumbral External Contact | 1977 April 18 at 07:33:32.8 UTC |
| First Umbral External Contact | 1977 April 18 at 08:41:17.9 UTC |
| First Central Line | 1977 April 18 at 08:43:51.9 UTC |
| First Umbral Internal Contact | 1977 April 18 at 08:46:26.6 UTC |
| First Penumbral Internal Contact | 1977 April 18 at 10:12:31.9 UTC |
| Equatorial Conjunction | 1977 April 18 at 10:18:48.0 UTC |
| Greatest Eclipse | 1977 April 18 at 10:31:29.9 UTC |
| Ecliptic Conjunction | 1977 April 18 at 10:36:13.6 UTC |
| Greatest Duration | 1977 April 18 at 10:40:30.0 UTC |
| Last Penumbral Internal Contact | 1977 April 18 at 10:50:48.6 UTC |
| Last Umbral Internal Contact | 1977 April 18 at 12:16:41.3 UTC |
| Last Central Line | 1977 April 18 at 12:19:17.4 UTC |
| Last Umbral External Contact | 1977 April 18 at 12:21:52.9 UTC |
| Last Penumbral External Contact | 1977 April 18 at 13:29:36.4 UTC |
| Parameter | Value |
|---|---|
| Eclipse Magnitude | 0.94492 |
| Eclipse Obscuration | 0.89288 |
| Gamma | −0.39903 |
| Sun Right Ascension | 01h45m03.2s |
| Sun Declination | +10°51'37.2" |
| Sun Semi-Diameter | 15'55.5" |
| Sun Equatorial Horizontal Parallax | 08.8" |
| Moon Right Ascension | 01h45m26.3s |
| Moon Declination | +10°30'41.9" |
| Moon Semi-Diameter | 14'50.4" |
| Moon Equatorial Horizontal Parallax | 0°54'28.0" |
| ΔT | 47.8 s |
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This eclipse is part of an eclipse season, a period, roughly every six months, when eclipses occur. Only two (or occasionally three) eclipse seasons occur each year, and each season lasts about 35 days and repeats just short of six months (173 days) later; thus two full eclipse seasons always occur each year. Either two or three eclipses happen each eclipse season. In the sequence below, each eclipse is separated by a fortnight.
| April 4 Ascending node (full moon) | April 18 Descending node (new moon) |
|---|---|
 |  |
| Partial lunar eclipse Lunar Saros 112 | Annular solar eclipse Solar Saros 138 |
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Eclipses in 1977
- A partial lunar eclipse on April 4.
- An annular solar eclipse on April 18.
- A penumbral lunar eclipse on September 27.
- A total solar eclipse on October 12.
Metonic
- Preceded by: Solar eclipse of June 30, 1973
- Followed by: Solar eclipse of February 4, 1981
Tzolkinex
- Preceded by: Solar eclipse of March 7, 1970
- Followed by: Solar eclipse of May 30, 1984
Half-Saros
- Preceded by: Lunar eclipse of April 13, 1968
- Followed by: Lunar eclipse of April 24, 1986
Tritos
- Preceded by: Solar eclipse of May 20, 1966
- Followed by: Solar eclipse of March 18, 1988
Solar Saros 138
- Preceded by: Solar eclipse of April 8, 1959
- Followed by: Solar eclipse of April 29, 1995
Inex
- Preceded by: Solar eclipse of May 9, 1948
- Followed by: Solar eclipse of March 29, 2006
Triad
- Preceded by: Solar eclipse of June 17, 1890
- Followed by: Solar eclipse of February 17, 2064
Solar eclipses of 1975–1978
This eclipse is a member of a semester series. An eclipse in a semester series of solar eclipses repeats approximately every 177 days and 4 hours (a semester) at alternating nodes of the Moon's orbit.[4]
| Solar eclipse series sets from 1975 to 1978 | ||||||
|---|---|---|---|---|---|---|
| Descending node | Ascending node | |||||
| Saros | Map | Gamma | Saros | Map | Gamma | |
| 118 | May 11, 1975 Partial |
1.0647 | 123 | November 3, 1975 Partial |
−1.0248 | |
| 128 | April 29, 1976 Annular |
0.3378 | 133 | October 23, 1976 Total |
−0.327 | |
| 138 | April 18, 1977 Annular |
−0.399 | 143 | October 12, 1977 Total |
0.3836 | |
| 148 | April 7, 1978 Partial |
−1.1081 | 153 | October 2, 1978 Partial |
1.1616 | |
Saros 138
This eclipse is a part of Saros series 138, repeating every 18 years, 11 days, and containing 70 events. The series started with a partial solar eclipse on June 6, 1472. It contains annular eclipses from August 31, 1598 through February 18, 2482; a hybrid eclipse on March 1, 2500; and total eclipses from March 12, 2518 through April 3, 2554. The series ends at member 70 as a partial eclipse on July 11, 2716. Its eclipses are tabulated in three columns; every third eclipse in the same column is one exeligmos apart, so they all cast shadows over approximately the same parts of the Earth.
The longest duration of annularity was produced by member 23 at 8 minutes, 2 seconds on February 11, 1869, and the longest duration of totality will be produced by member 61 at 56 seconds on April 3, 2554. All eclipses in this series occur at the Moon’s descending node of orbit.[5]
| Series members 20–41 occur between 1801 and 2200: | ||
|---|---|---|
| 20 | 21 | 22 |
 January 10, 1815 |
 January 20, 1833 |
 February 1, 1851 |
| 23 | 24 | 25 |
 February 11, 1869 |
 February 22, 1887 |
 March 6, 1905 |
| 26 | 27 | 28 |
 March 17, 1923 |
 March 27, 1941 |
 April 8, 1959 |
| 29 | 30 | 31 |
April 18, 1977 |
 April 29, 1995 |
 May 10, 2013 |
| 32 | 33 | 34 |
 May 21, 2031 |
 May 31, 2049 |
 June 11, 2067 |
| 35 | 36 | 37 |
 June 22, 2085 |
 July 4, 2103 |
 July 14, 2121 |
| 38 | 39 | 40 |
 July 25, 2139 |
 August 5, 2157 |
 August 16, 2175 |
| 41 | ||
 August 26, 2193 | ||
Metonic series
The metonic series repeats eclipses every 19 years (6939.69 days), lasting about 5 cycles. Eclipses occur in nearly the same calendar date. In addition, the octon subseries repeats 1/5 of that or every 3.8 years (1387.94 days). All eclipses in this table occur at the Moon's descending node.
| 22 eclipse events between September 12, 1931 and July 1, 2011 | ||||
|---|---|---|---|---|
| September 11–12 | June 30–July 1 | April 17–19 | February 4–5 | November 22–23 |
| 114 | 116 | 118 | 120 | 122 |
 September 12, 1931 |
 June 30, 1935 |
 April 19, 1939 |
 February 4, 1943 |
 November 23, 1946 |
| 124 | 126 | 128 | 130 | 132 |
 September 12, 1950 |
 June 30, 1954 |
 April 19, 1958 |
 February 5, 1962 |
 November 23, 1965 |
| 134 | 136 | 138 | 140 | 142 |
 September 11, 1969 |
 June 30, 1973 |
April 18, 1977 |
 February 4, 1981 |
 November 22, 1984 |
| 144 | 146 | 148 | 150 | 152 |
 September 11, 1988 |
 June 30, 1992 |
 April 17, 1996 |
 February 5, 2000 |
 November 23, 2003 |
| 154 | 156 | |||
 September 11, 2007 |
 July 1, 2011 | |||
Tritos series
This eclipse is a part of a tritos cycle, repeating at alternating nodes every 135 synodic months (≈ 3986.63 days, or 11 years minus 1 month). Their appearance and longitude are irregular due to a lack of synchronization with the anomalistic month (period of perigee), but groupings of 3 tritos cycles (≈ 33 years minus 3 months) come close (≈ 434.044 anomalistic months), so eclipses are similar in these groupings.
| Series members between 1801 and 2200 | ||||
|---|---|---|---|---|
 August 28, 1802 (Saros 122) |
 July 27, 1813 (Saros 123) |
 June 26, 1824 (Saros 124) |
 May 27, 1835 (Saros 125) |
 April 25, 1846 (Saros 126) |
 March 25, 1857 (Saros 127) |
 February 23, 1868 (Saros 128) |
 January 22, 1879 (Saros 129) |
 December 22, 1889 (Saros 130) |
 November 22, 1900 (Saros 131) |
 October 22, 1911 (Saros 132) |
 September 21, 1922 (Saros 133) |
 August 21, 1933 (Saros 134) |
 July 20, 1944 (Saros 135) |
 June 20, 1955 (Saros 136) |
 May 20, 1966 (Saros 137) |
April 18, 1977 (Saros 138) |
 March 18, 1988 (Saros 139) |
 February 16, 1999 (Saros 140) |
 January 15, 2010 (Saros 141) |
 December 14, 2020 (Saros 142) |
 November 14, 2031 (Saros 143) |
 October 14, 2042 (Saros 144) |
 September 12, 2053 (Saros 145) |
 August 12, 2064 (Saros 146) |
 July 13, 2075 (Saros 147) |
 June 11, 2086 (Saros 148) |
 May 11, 2097 (Saros 149) |
 April 11, 2108 (Saros 150) |
 March 11, 2119 (Saros 151) |
 February 8, 2130 (Saros 152) |
 January 8, 2141 (Saros 153) |
 December 8, 2151 (Saros 154) |
 November 7, 2162 (Saros 155) |
 October 7, 2173 (Saros 156) |
 September 4, 2184 (Saros 157) |
 August 5, 2195 (Saros 158) | |||
Inex series
This eclipse is a part of the long period inex cycle, repeating at alternating nodes, every 358 synodic months (≈ 10,571.95 days, or 29 years minus 20 days). Their appearance and longitude are irregular due to a lack of synchronization with the anomalistic month (period of perigee). However, groupings of 3 inex cycles (≈ 87 years minus 2 months) comes close (≈ 1,151.02 anomalistic months), so eclipses are similar in these groupings.
| Series members between 1801 and 2200 | ||
|---|---|---|
 August 17, 1803 (Saros 132) |
 July 27, 1832 (Saros 133) |
 July 8, 1861 (Saros 134) |
 June 17, 1890 (Saros 135) |
 May 29, 1919 (Saros 136) |
 May 9, 1948 (Saros 137) |
April 18, 1977 (Saros 138) |
 March 29, 2006 (Saros 139) |
 March 9, 2035 (Saros 140) |
 February 17, 2064 (Saros 141) |
 January 27, 2093 (Saros 142) |
 January 8, 2122 (Saros 143) |
 December 19, 2150 (Saros 144) |
 November 28, 2179 (Saros 145) |
|
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