Astronomy Answers
Eclipses and the Saros

1. Lunar Eclipses

A lunar eclipse occurs when the Moon passes through the shadow of the Earth. For more basic information about lunar eclipses, see the Eclipses Page from the AstronomyAnswerBook.

If the Moon stands opposite the Sun in the sky then it is Full Moon. If there is a lunar eclipse, then the Moon goes through the shadow of the Earth, which is directly opposite the Sun, so it has to be Full Moon when there is a lunar eclipse. The other way around, there is not a lunar eclipse every Full Moon, because the Moon does not move exactly along the ecliptic, but can get up to about 5 degrees above or below the ecliptic, because of the tilt (inclination) of the orbit of the Moon around the Earth.

There is only a lunar eclipse if the Moon, when it is full, is also close enough to the ecliptic. The two points where the orbit of the Moon crosses the ecliptic are called the nodes of the lunar orbit. There is a lunar eclipse only if the Moon, at Full Moon, is close enough to a node of its orbit.

There are on average 29.530588 days between a Full Moon and the next Full Moon, and on average 13.606111 days between one passage through a node of the Moon's orbit and the next one. That first period is called the synodical month, and the second period is half of the draconic month.

It turns out that 223 synodical months is nearly equal to 242 draconic months, namely both almost equal to 6585 1/3 days or about 18 years and 11 days. That long after a lunar eclipse you can expect another one. This period of 223 synodical months, which is nowadays called a saros, was already used 2500 years ago by Babylonian astronomers to predict lunar eclipses. There are 223 independent series of Full Moons in which each next Full Moon comes one saros after the previous one. We give all Full Moons in such a series the same saros number, which is between 1 and 223 (inclusive).

During a lunar eclipse, the Moon gets dark, and everyone who can see the Moon then can see the lunar eclipse. For this, the Moon has to be above the horizon, and the weather must cooperate. For an average location, you can expect that slightly more than one half of all lunar eclipses are visible (if the weather cooperates).

2. Predict Lunar Eclipses Yourself

Here is a method to predict eclipses of the Moon yourself. During the coming years, lunar eclipses occur for Full Moons of which the saros number is one of the numbers of 109 through 150. If the saros number is between 121 and 137 (inclusive), then it is a total eclipse. If the saros number is between 109 and 120 or between 138 and 150, then there is a penumbral or partial eclipse of the Moon.

2.1. Per Full Moon

You can calculate the saros number of a Full Moon as follows: The first Full Moon of 2003 (on 17 January) has the saros number 192. This is outside of the interval 109 − 150 so there is no lunar eclipse then. For each next Full Moon you must add 38 to the saros number. If the saros number gets greater than 223, then you should immediately subtract 223.

For example, the saros number of the second Full Moon of 2003 is equal to 192 + 38 = 230, but that is greater than 223 so we subtract 223 and find 7, so no lunar eclipse. The saros number of the third Full Moon is 7 + 38 = 45, so yet again no eclipse. We continue and find 45 + 38 = 83 for the fourth Full Moon and 83 + 38 = 121 for the fifth Full Moon. That last number is in the interval of total lunar eclipses (121 − 137), so the fifth Full Moon of 2003 has a total lunar eclipse. For the sixth Full Moon the saros number is 121 + 38 = 159 which is too high for a lunar eclipse.

To help you along, here is a table that shows the dates and saros numbers of the first Full Moon of the years 2003 − 2010. The column "FM" gives the ordinal number of the first Full Moon of that year in a series that starts with number 1 for the first Full Moon of the year 2000. The column "Jan" gives the date in January which has the first Full Moon of the year. The column "Saros" lists the corresponding saros number.

Table 1: "Saros Numbers 2003−2021 for the Moon"

2.2. Per Lunar Eclipse

Once you've found a lunar eclipse, there will be a next one 1, 5, or 6 Full Moons later, belonging to a saros number that is 38 greater, 33 less, or 5 greater, respectively, than the saros number of the previous eclipse. The recipe is therefore as follows:

1. If the saros number of the lunar eclipse is between 109 and 112 (inclusive), then there are additional non-total lunar eclipses 1 and 6 Full Moons later, with saros numbers that are 38 greater and 5 greater, respectively.
2. If the saros number is between 113 and 115, then there is another non-total lunar eclipse 6 Full Moons later, for a saros number that is 5 greater.
3. If the saros number is between 116 and 132, then there is a total lunar eclipse 6 Full Moons later, with a saros number that is 5 greater.
4. If the saros number is between 133 and 141, then there is a non-total lunar eclipse 6 Full Moons later, with a saros number that is 5 greater.
5. If the saros number is between 142 and 145, then there are additional non-total lunar eclipses 5 and 6 Full Moons later, with saros numbers that are 33 less and 5 greater, respectively.
6. If the saros number is between 146 and 150, then there is another non-total lunar eclipse 5 Full Moons later, with a saros number that is 33 less.

3. Saros Series for the Moon

One saros (about 18 years and 11 days) after a Full Moon there is another Full Moon with the same saros number. If there is a lunar eclipse during the first Full Moon, then there will almost always be another lunar eclipse one saros later, which will look a lot like the previous one (except for its time of day). This way, you find a series of lunar eclipses with the same saros number. I call such a series of eclipses a saros series.

There is a small difference between the 223 synodical months of a saros and the 242 draconic months to which it is almost equal. Because of this small difference, each next eclipse from a particular saros series is yet slightly different from the previous one. You can point at the best eclipse of each saros series, for which the times of passage through the node of the orbit of the Moon is closest to the time of Full Moon. If you move away from that eclipse, either forward in time or backward, one saros at a time, then the Full Moon and passage through the node get increasingly out of step, until they get so far out of step that there are no eclipses anymore. Because of this, each saros series contains only a limited number of eclipses, immediately before of after which there are no eclipses. In other words: The interval of saros numbers for which there are eclipses shifts up by one about every 31 years.

Most saros series contain 71 − 73 eclipses of the Moon during a period of about 1300 years, but some may have up to 84 eclipses spanning 1500 years.

Data for saros series (of lunar eclipses) that end after 1899 and start before 2060 are given in the next table. The data are derived from a list of lunar eclipses between the years −1999 and 3000 prepared by Fred Espenak. The column "Saros" gives the saros number. The columns "First" and "Last" give the dates of the first and last eclipses of the series. If "Last" says "> 3000", then that saros series lasts beyond the year 3000. The column "#" shows the number of lunar eclipses in the saros series (through the year 3000). The columns "N", "P", and "T" count the number of penumbral lunar eclipses (the Moon passes through the penumbral shadow of the Earth only), partial umbral lunar eclipses (the Moon passes partly through the umbral shadow of the Earth), and total lunar eclipses (the Moon passes wholly through the umbral shadow of the Earth) through the year 3000. The column "Next" shows the date of the next lunar eclipse of that series (after 1 January 2003), and the column "t" shows the type of that lunar eclipse.

Table 2: "Lunar Eclipses 2003−2021"

 # 

For example, look at saros series 126. The first lunar eclipse of that series occurred in the year 1210, and the last one will happen in the year 2490. Saros series 126 has 72 lunar eclipses, of which 31 are penumbral, 27 partial, and 14 total. The next eclipse from that series (after 1 January 2003) happens on 9 November 2003 and is a total eclipse of the Moon.

You can use saros numbers to predict reasonably accurately when there will be lunar eclipses, but not whether they will be visible from a particular location (such as your street). For that, you need to know not just your location but also the time of the eclipse, and that time can be determined accurately enough only through lengthy calculations.

4. Solar Eclipses

A solar eclipse happens when the shadow of the Moon passes across the Earth. As seen from the Earth, the Moon then passes in front of the Sun. For more basic information about solar eclipses, see the Eclipses Page from the AstronomyAnswerBook.

5. Predict Your Own Solar Eclipses

The method which I gave earlier for predicting lunar eclipses can also be used to predict solar eclipses, if you change a few numbers. For solar eclipses there are saros numbers, just like for lunar eclipses, but a solar eclipse with a certain saros number is not related to a lunar eclipse with the same saros number. The range of saros numbers for which there are solar eclipses during the coming years is 117 through 156. The range of saros numbers for which only total or annular solar eclipses occur is 126 through 140, but there are also a few such eclipses outside of that range.

5.1. Per New Moon

You can calculate the saros number of a New Moon as follows: The first New Moon of 2003 (on 3 January) has saros number 180. This is outside of the range 117 − 156 of saros numbers with solar eclipses in the near future, so there is no solar eclipse at the first New Moon of 2003. Just like for lunar eclipses you should add 38 to the saros number for each next New Moon, and immediately subtract 223 if the number gets greater than 223.

In this way you find that the saros numbers of the first seven New Moons of 2003 are equal to 180, 218, 33, 71, 109, 147, and 185. The saros numbers 109 and 147 are inside the proper range, so there are solar eclipses during the fifth and sixth New Moons of 2003.

Here is a table showing the dates and saros numbers of the first New Moon of the years 2003 − 2021. The column "NM" gives the ordinal number of the first New Moon of the year in a series that starts with number 1 for the first New Moon of the year 2000. The column "Jan" gives the date in January of the first New Moon of the year. The column "Saros" provides the corresponding saros number.

Table 3: "Saros Numbers 2003−2021 for the Sun"

5.2. Per Solar Eclipse

If you've found a solar eclipse, then the next one will occur 1, 5, or 6 New Moons later and will have a saros number that is 38 greater, 33 less, or 5 greater, respectively, than that of the previous solar eclipse. The recipe is as follows:

1. If the saros number of the solar eclipse is 117 or 118, then there is another (likely partial) solar eclipse 1 and 6 New Moons later, with saros numbers that are 38 greater and 5 greater, respectively.
2. If the saros number is 119 or 120, then there is another (probably partial) solar eclipse 6 New Moons later, with a saros number that is 5 greater.
3. If the saros number is between 121 and 143, then there is a total or annular solar eclipse 6 New Moons later, with a saros number that is 5 greater.
4. If the saros number is between 144 and 149, then there is a (likely partial) solar eclipse 6 New Moons later, with a saros number that is 5 greater.
5. If the saros number is 150 or 151, then there are (likely partial) solar eclipses 5 and 6 New Moons later, with saros numbers that are 33 less and 5 greater, respectively.
6. If the saros number is between 152 and 156, then there is another (probably partial) solar eclipse 5 New Moons later, with a saros number that is 33 less.

6. Saros Series for the Sun

Solar eclipses can be divided into saros series just like lunar eclipses can. Most saros series for solar eclipses contain 72 solar eclipses during a period of about 1300 years, but occasionally a saros series contains up to 86 solar eclipses spanning 1550 years.

The next table contains information about saros series (of solar eclipses) that end after 1899 and begin before 2060. The information is derived from a list of solar eclipses between the years −1999 and 3000 that was prepared by Fred Espenak. The column "Saros" gives the saros number. The columns "First" and "Last" provide the dates of the first and last solar eclipses of that saros series. If "Last" says "> 3000" then that saros series continues beyond the year 3000. The column "#" gives the number of solar eclipses in the saros series (through the year 3000). The columns "P", "A", "H", and "T" count the number of partial solar eclipses (the Moon covers only part of the Sun), annular solar eclipses (the Moon may make the Sun look like a bright ring), hybrid solar eclipses (in some places as an "A" and in other places as a "T"), and total solar eclipses (the Moon can cover the whole Sun) through the year 3000. The column "Next" gives the date of the next solar eclipse of the series (after 1 January 2003), and the column "t" gives the type of that solar eclipse.

Table 4: Solar Eclipses 2003−2021

 # 

You can use saros numbers to predict reasonably well whether there will be a solar eclipse, but not whether that eclipse will be visible from a certain location (like your street). For that you need to know not just your location but also the time of the eclipse, which can be determined accurately enough only through lengthy calculations.

6.1. Calculate It Yourself

There is a calculation page for prediction periods such as the saros.

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//aa.quae.nl/en/saros.html;
Last updated: 2021-07-19