An Eclipse Primer

(C) 1999, 2008 Ted Saker, Jr.

Three conditions bring about total solar eclipses. The Moon's phase (new), coinciding with the Moon passing through a node (a syzygy, or the point where the Moon's orbit intersects the ecliptic), at the optimum distance from the Earth, results in the Moon’s umbra touching the Earth’s surface.

Each eclipse belongs to a family. Each family is known by a given "Saros" number, named after the Greek astronomer who discovered the periodicity of eclipses. Each eclipse in a Saros family shares many similar characteristics, including the length of totality at various points along the shadow’s path, the shape and width of the shadow’s path, and the places on Earth where the shadow’s path tracks across. The August 11, 1999 eclipse belonged to the family Saros 145. The next eclipse in this family will occur on August 21, 2017, and will track across the United States (this is the one North Americans are waiting for!).

Saros families are numbered according to the direction the Moon is passing in its orbit during an eclipse, among other criteria. Keep in mind that the plane of the Moon’s orbit is inclined approximately 5° to the plane of the Earth’s orbit. This is why we do not experience total eclipses on a monthly basis (darn it!). When the Moon passes through a node, it is either traveling to points either above or below the plane of the Earth’s orbit (the ecliptic). An even number Saros designation means that the Moon is passing through an ascending node, and the shadow generally traces a positively-sloped arc (west to east) on the Earth’s surface. An odd number Saros designation means the Moon is passing through a descending node, and the shadow generally traces a negatively-sloped arc (west to east) on the Earth’s surface.

The periodicity of solar eclipses depends upon two lunar orbital cycles coinciding with the Moon passing through a node. First, a new moon occurs on average every 29.530588 days. The Moon’s average orbital period, perigee to perigee, is 27.554548 days. These cycles repeat every 18 years, 11 1/3 days; or 6585.3211 days; or 233 new moons, approximately 239 perigees and 242 nodes. Every eclipse in a Saros family shares the same 18 year, 11.33 day cycle. The great eclipses of June 30, 1973 and July 11, 1991 (Saros 136), which presented over and slightly under seven minutes of totality, respectively, were precisely 18 years, 11.33 days apart. The next total solar eclipse of family Saros 136 should occur on July 22, 2009, precisely 18 years, 11.33 days after that.

A second eclipse cycle has solar eclipses following the main ones by 12 new moons (354 days) on June 19, 1974 and June 30, 1992, and preceding them by 12 new moons (354 days) on July 11, 1972 and July 22, 1990. For almost 3000 years, astronomers have known of a third eclipse cycle. A correlation in a 19 solar-lunar year cycle (consisting of 235 new moons and 255 nodes) separates total eclipses belonging to different Saros families by exactly 19 years. Notice that the family designations also fall into a pattern. The family number of each successive eclipse in the 19 year cycle is ten more than its predecessor as seen in the Table of Eclipse Cycles appearing below.

Eclipse families have beginnings and endings. Some families are active, some have ended, and some have yet to start. Saros 145, the family of the upcoming "big one" that will bisect the U.S., first made its appearance on January 4, 1639 as a very partial eclipse. The first total eclipse of the family occurred on June 29, 1927, and yielded a whopping 50 seconds of totality. The longest total eclipse of this family should occur on June 25, 2522, yielding 7 minutes, 12 seconds of totality, which is close to the theoretical maximum amount. The last total solar eclipse in the family should occur on September 9, 2648, producing 2 minutes 49 seconds of totality. Saros 145 family should end on April 17, 3009, as it started, with another very partial eclipse. All told, the Saros 145 family encompasses 77 eclipses spanning 1,370.3 years.

Eclipse cycles also have beginnings and endings. For example:


 

2/26/98 T (130)

+ 354 days

2/16/99 A (140)

+ 354 days

2/5/00 P (150)

8/22/98 A (135)

+ 354 days

8/11/99 T (145)

+ 354 days

7/31/00 P (155)

But:


7/11/99 ? (107?)

+ 354 days

7/1/00 P (117)

+ 354 days

6/21/01 T (127)



Notice that Saros 117 is the smallest Saros number on the table. The 19 year and 354 day cycles start with the lowest Saros family number; thus, there was no solar eclipse of any type 354 days before the partial eclipse of July 1, 2000. Also, Saros family 107 (ten less than Saros 117) ended on April 5, 1837, and Saros family 116 ended on July 22, 1971. The 354 day cycle coincidentally began with the end of the second millennium.

The Table of Eclipse Cycles, Figure 1, reveals that the 18 year, 11.33 day and 354 day cycles separate families of Saros 126, 136, and 146. Note that not all the eclipses related in the 354 day cycle are total eclipses. Some are partials. Other groupings of Saros families follow this identical pattern. All the different families are interlaced in the 354 day cycle, while eclipses of the same family are separated by the 18 year, 11.33 day cycle. Figure 2 illustrates the 19 year cycle. Both figures of the Table of Eclipse Cycles illustrates the interrelation between eclipse families and types. The Saros families used in this example are all total eclipses.

Table of Eclipse Cycles

Fig. 1:
 

7/11/72 (126)

+ 18 years 11.33 days

7/22/90 (126)

+ 18 years 11.33 days

8/1/08 (126)

+ 354 days


+ 354 days


+ 354 days

6/30/73 (136)

+ 18 years 11.33 days

7/11/91 (136)

+ 18 years 11.33 days

7/22/09 (136)

+ 354 days


+ 354 days


+ 354 days

6/20/74 (146)

+ 18 years 11.33 days

6/30/92 (146)

+ 18 years 11.33 days

7/11/10 (146)


 

Fig. 2
 

7/10/72 (126) + 19 years =>

7/11/91 (136) + 19 years =>

7/11/10 (146)

7/22/90 (126) + 19 years =>

7/22/09 (136) + 19 years =>

7/22/28 (146)

6/30/73 (136) + 19 years =>

6/30/92 (146) + 19 years =>

7/1/11 (156)




Table of Past, Present and Future Eclipses

Eclipse predictions by Fred Espenak, NASA/GSFC. For lots of information, maps, and details about solar and lunar eclipses, and to see your tax dollars doing good work, see Fred’s excellent Eclipse pages at: http://sunearth.gsfc.nasa.gov/eclipse/. To see the extent of one man's efforts to stand beneath the Moon's shadow, go to the Eclipse Chasers site.

Key: T=Total (duration), A=Annular, A/T=Annular/Total, P=Partial


Past

Present

Future

Saros

6/30/73 T (7:04)

7/11/91 T (6:58)

7/22/09 T (6:39)

136

12/24/73 A

1/4/92 A

1/15/10 A

141

6/20/74 T (5:09)

6/30/92 T (5:26)

7/11/10 T (5:20)

146

12/13/74 P

12/24/92 P

1/4/11 P

151

5/11/75 P

5/21/93 P

6/1/11 P

118

11/3/75 P

11/13/93 P

11/25/11 P

123

4/29/76 A

5/10/94 A

5/20-21/12 A

128

10/23/76 T (4:46)

11/3/94 T (4:28)

11/13/12 T (4:02)

133

4/18/77 A

4/29/95 A

5/9-10/13 A

138

10/12/77 T (2:37)

10/24/95 T (2:15)

11/3/13 A/T (1:39)

143

4/7/78 P

4/17/96 P

4/29/14 P

148

10/2/78 P

10/12/96 P

10/23/14 P

153

2/26/79 T (2:49)

3/9/97 T (2:54)

3/20/15 T (2:47)

120

8/22/79 P

9/2/97 P

9/13/15 P

125

2/16/80 T (4:12)

2/26/98 T (4:13)

3/9/16 T (4:09)

130

8/10/80 A

8/22/98 A

9/1/16 A

135

2/4/81 A

2/16/99 A

2/26/17 A

140

7/31/81 T (2:06)

8/11/99 T (2:27) *

8/21/17 (2:40) *

145

1/25/82 P

2/5/00 P

2/16/18 P

150

6/21/82 P

7/1/00 P

7/13/18 P

117

7/20/82 P

7/31/00 P

8/11/18 P

155

12/15/82 P

12/25/00 P

1/6/19 P

122

6/11/83 T (5:11)

6/21/01 T (4:57)

7/2/19 T (4:33)

127

12/4/83 A

12/14/01 A

12/26/19 A

132

5/30/84 A

6/10/02 A

6/21/20 A

137

11/22/84 T (1:51)

12/4/02 T (2:04)

12/14/20 T (2:10)

142

5/19/85 P

5/31/03 A

6/10/21 A

147

11/12/85 T (1:58)

11/23/03 T (1:57)

12/4/21 (2:06)

152

4/9/86 P

4/19/04 P

4/30/22 P

119

3/29/87 A/T

4/8/05 A/T

4/20/23 A/T

129

9/22/87 A

10/3/05 A

10/14/23 A

134

3/17/88 T (3:46)

3/29/06 T (4:07)

4/8/24 T (4:27)`**

139

9/11/88 A

9/22/06 A

10/2/24 A

144

3/7/89 P

3/19/07 P

3/29/25 P

149

8/31/89 P

9/11/07 P

9/21/25 P

154

1/26/90 A/T

2/7/08 A

2/17/26 A

121

7/22/90 T (2:36)

8/1/08 T (2:28)

8/12/26 T (2:18)

126

1/16/91 A

1/26/09 A

2/6/27 A

131

7/11/91 T (6:58)

7/22/09 T (6:39)

8/2/27 T (6:23)

136



My first eclipse chase, all of 120 miles from ColumbusOhio to Toledo, where we enjoyed over six minutes of annularity under crystal clear Ohio skies at the Holiday Inn, Perrysburg.

My first total eclipse, observed from the southern Caribbean aboard the MS Veendam (See Jul. 99 S&T).

* My second total eclipse. The next eclipse in this Saros family is the big one of 2017, visible across the United States.

** The even bigger one of 2024, observable from my home in ColumbusOhio (weather permitting, but there's a factual basis for the old saying about April showers especially in Ohio, and TS Eliot wrote about April being the cruelest month).

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