Results from the Lunar Eclipse

After the eclipse

This text incorporates the results from observations of the Lunar Eclipse on September 27, 1996, by Astronomy On-Line Groups. It also contains some general remarks about such eclipses and navigation on the seas. Before you start reading, it is strongly recommended that you read the general text about lunar eclipses, which was prepared before the eclipse took place.

This collaborative project within the Astronomy On-Line Programme is concerned with the observation of a Total Lunar Eclipse, which took place when the Moon entered the Earth's shadow on that date. During this event, less sunlight reached the Moon and its appearance changed. During a Total Lunar Eclipse, the Moon is entirely inside the Earth's shadow; during a Partial Lunar Eclipse, only part of the Moon is inside.

A Lunar Eclipse always happens at the time of full moon, when the Sun, Earth and Moon are (nearly) aligned.

The following text contains a review of the Astronomy On Line observations reported by Astronomy On-Line Groups. In addition, it tells you how you may determine the longitude difference between Europe and America by means of these observations!

The eclipse

This is a diagram that shows how the Moon passed through the Earth's shadow on September 27.

Here, the time is indicated in Universal Time (UT), which is equal to British Winter Time. If you were located in another timezone, you must correct for the time difference in order to know the local time of the eclipse. For instance, for observers in Central Europe which still used Central European Summer (Daylight Saving) Time (UT + 2 hours), the Moon left totality at 05 hours 29.4 minutes CEST.

Many observers commented on the beautiful sight of the planet Saturn which was located near the eclipsed Moon.

Where was the eclipse visible?

Lunar Eclipses are global events which can be observed from many geographical regions on the Earth. Unfortunately, quite a few European countries did suffer from clouded sky, but here is a review of some of the reports from those who were not clouded out:

The Copper-red Moon looked like a Billiard Sphere made of Ivory - lit by natural fire from one side. (Gabor S.-Nagy, Budapest, Hungary).

The Sept.1997 Lunar Eclipse, as registered by Antonio Cidadao, AOL Portugal.

Our students expressed having had an experience for lifetime (Henry Noergaard, Noerre Gymnasium, Copenhagen, Denmark).

The Eclipse was sensational over Santiago, with clear sky and a perfect sight (Astronomy On-Line Chile, Jorge and Rodrigo).

A wonderful quiet night, the grashoppers were singing, while the dark-red moon was hanging slightly above the planet Saturn. (Bent Klarmark, Kettinge Youth School, Denmark).

The night was wonderful, the sky was clear and the constellations were finely visible. This eclipse has both intrigued and fascinated us. (Apollo Group, Bellinzona, Switzerland.).

While observers in Europe had to stay up into the late night in order to observe this eclipse, Stig Linander from Seattle, as well as the California Lower Lake High School had a most comfortable evening view. Stig writes to Astronomy On-Line:

The Moon was already totally eclipsed during sunset - 19 hours 19 min local time. While rising, it gave a strange ghostlike light above the impressive Cascade Mountains. Followed it until the end - Earth atmosphere absorption seemed to be low - this was the brightest eclipse I have ever seen.

The accurate eclipse data from Lower Lake High School are given together with other European data at the end of this text.

More student images may be found, for instance at the

Norwegian Astronomy On-Line site. These are marvellous eclipse photos, taken by the the students at the Norwegian Highschool `Tromsdalen Gymnasium'. Click here.

You may also like to inspect the fine results by a Danish student, now available the Danish Astronomy On-Line site. She was able to determine the distance to the Moon by means of the method described in an article by Dennis di Cicco (Sky and Telescope, Dec. 1992, p.704). Click here.

Below, we take a closer look at some of the obtained Astronomy On-Line data. In particular, we will apply these data in order to estimate the longitude difference between Europe and the American continent.

A Math Exercise related to data obtained during this Lunar Eclipse.

A number of Astronomy On-Line Groups had good sky conditions which allowed both a determination of the time at the start of totality and also for `Local Noon', that is when the Sun is seen in the direction of South).

Here follow some of the reported observations, sorted by geographical longitude. A few of these timings may suffer from minor time offsets (if the watch was not entirely correct). However, if they were measured with the same watch, the time DIFFERENCE between eclipse and local noon will be correct.

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Quiet Sun Group (Hersby Skola, Lidingoe near Stockholm, Sweden) observed at 18 Deg 05' East, 59 Deg 29' North. They found:

Local Noon : 12:39 CEST

Start of totality - "Last Light": 04:19 CEST

Time Difference : 15 hours 40 minutes.

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Soroe Akademi Group (Soroe, Denmark) observed at 11 Deg, 33' East, 55 Deg 26' North:

Local Noon : 13:05 - measured by sextant

Start of totality - "Last Light": 4:17 CEST

Time Difference : 15 hours 12 minutes.

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Apollo Group (Lyceo Cantonale, Bellinzona, Switzerland) observed at 9 Deg 03' East, 46 Deg 13' North:

Local Noon : 13:07 CEST

Start of totality - "Last Light": 04:12 CEST

Time Difference : 15 hours 05 minutes

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Club Astro Group (Lycee Jean Moulin, Draguignan, France) observed at 6 Deg 28' East, 43 Deg 32' North:

Local Noon : 13:30 CEST

Start of totality : 04:20

Time Difference : 14 hours 50 minutes

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Lower Lake High School (Northern California, USA) observed from 122 Deg 40 ' West, 39 Deg 00' North:

Local Noon : 13:02 PST (Pacific Standard Time = UT - 7 hours)

Start of totality - "Last Light": 19:19

Time Difference: 6 hours 17 minutes.

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The timings indicated above tell the time, when the last spot on the Moon that was directly illuminated by the Sun, suddenly disappeared.

Comparing data obtained within Europe

Two groups made particular careful measurements of both eclipse timings, and the local noon:

The Danish Soroe Group observed at longitude 11 Deg, 33' East, and their measured time interval between local noon and begin of total eclipse was : 15 hours 12 minutes.

Club Astro in France observed at longitude 6 Deg 28' East: - and measured a time difference of 14 hours 50 minutes.

So, the interval between local noon and the eclipse was 22 minutes longer for the group in Denmark than for the group in France.

Remember that 24 hours timing difference correspond to a longitude difference of 360 Deg.

Try this exercise: Show that this Danish-French Time difference corresponds to 5,5 Deg. Compare with actual geographical position. Show that the observed East-West error is about half a degree.

Close to Equator, half a degree in longitude would correspond to an error of approximately 60 km only! Verify this! At the latitude of the Danish and French observers, the error is even less!

Combining measurements from America and Europe

In Stockholm, the data obtained by the Swedish group, indicated above, show the total lunar eclipse begins 15 hours 40 minutes after Local Noon.

In California, the total eclipse begins approximately 6 hours 17 minutes after Local Noon.

Thus there is a clear timing difference of 15h 40m - 6h 17 m = 9h 23m.

Here is another exercise: When 24 hours correspond to a longitude difference of 360 degrees, show that this timing difference equals about 140 Deg in geographical longitude. Compare the longitudes indicated for the sites of the observers above and also with your map. Discuss, how you could estimate the size of our Globe.

Navigation and the longitude problem

You have of course noted that, in the data above, there is a clear relation between longitude position and eclipse timings. In addition, the involved Astronomy On-Line Groups have actually repeated the Columbus/Plinius experiment, see the general text about lunar eclipses, enabling rather precise estimates of longitude differences.

You are probably also aware that today all longitudes are compared not to Sweden, not to Denmark or France, but to the longitude of the old Greenwich Observatory, near London.

This September's lunar eclipse gave us the possibility of timing our watches, of comparing time differences by means of the same astronomical event.

However, these eclipses are relatively rare. There are typically only about 50 lunar eclipses during a life-time. Thus, lunar eclipses were not frequent enough to provide large-scale mapping of the Earth in old days. Another well-timed astronomical phenomenon, that could also be observed at the same time (simultaneously) from many sites the Earth, was needed for this.

For this reason, Galileo once suggested sailors should observe the Moons of Jupiter, since these actually eclipse each other quite often (this is referred to as mutual events by astronomers). This would be a quite useful method for the measurement of longitudes, provided you had accurate tables of the times these moons were eclipsed.

It was during the preparation of such tables for sailors that Ole Roemer made his most significant discovery in 1676: the finite speed of light. If you want to know more about this, try to read the article in the EAAE Newsletter #3.

However, this particular method required a lot of skill, and was practically useless on a ship rolling on a stormy ocean. So, sailors needed another method of timing. A most useful instrument was invented by a British Watch Maker, James Harrison, in 1735.

Harrisons invention, a chronometer, was a most precise watch, which kept the time irrespective of the ship's movements and it also worked in stormy conditions.

During a very stormy month-long voyage to Jamaica and back again to London (it was reported that "even the Captain became seasick"), a watch of this type was found to keep its original time to within an error of only +/- 30 seconds!

Try this Math Exercise: 24 hours corresponds to the circumference of the Earth, that is 40 000 km. What distance error would a timing error of +/- 30 seconds correspond to?

Now the British sailors had a most precise timing method, and they no longer needed any eclipses. In fact, Harrison's invention gave the British Admiralty the best chances of determining local noons and accurate longitudes. The longitudes thus measured by Sir James Coook in the second half of the 18th century resulted in the most precise World Maps ever made.

This is the main reason why - still today - the longitude starts at 0 Deg in Greenwich, the observatory of the British Admiralty, outside London.

A Note about the Moon's colour

As observed by many Astronomy On-Line participants, the eclipsed Moon was not completely dark. They were able to see it all the time during the eclipse.

Its colour was dark red like it is shown on the figure above. This strange colour was the result of sunlight being refracted when it passed through the Earth's upper atmosphere. Some of this light reached the Moon. This was why the Moon was not completely dark, even when it was completely inside the Earth's geometric shadow.

Before the 1960s, Lunar Eclipses were therefore also a valuable means to investigate the conditions in the upper atmosphere of the Earth. For instance, due to the injected high-altitude dust in the Earth's atmopshere, the eclipsed Moon has a different colour during years of heavy volcanic eruptions (Krakatau, Mount St. Helens, etc.), when compared to years with less volcanic activity.

This year, the Moon was comparatively bright red, indicating that there was comparatively little dust in the high layers of the Earth's atmosphere. This may have changed later, however, due to the extensive volcanic activity on Iceland in early October.

Thank You!

We appreciate all your efforts, also of the groups which were so unlucky to be clouded out!. We hope that you also participate in the observations of Solar Eclipse on October 12, 1996!

This project was prepared by the EAAE European Student Project Group of the European Association for Astronomy Education (EAAE):