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This collaborative programme is concerned with the observation of a
Lunar Eclipse, that is when the Moon enters the Earth's shadow.
During this event, less sunlight reaches the Moon and its appearance
changes. 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 describes the Total Lunar Eclipse of September 27, 1996, and the historical importance of lunar eclipses when mapping the Earth.
It also contains suggestions for your own observations, and tells you how you may determine your geographical longitude by comparing your observations with those obtained by other Astronomy On-Line Groups!
Addition (October 7, 1996): The first results of the observations by Astronomy On-Line Groups are now available!
Solar eclipses often appear just before or after a total lunar eclipse. This happens in September-October 1996 when a total lunar eclipse takes place, 14 days prior to the Partial Solar Eclipse on October 12. This is a diagram that shows how the Moon passes 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 are located in another timezone, you must correct for the time difference in order to know the local time of the eclipse. For instance, observers in Central Europe must remember that the usual Summer (Daylight Saving) Time differs from Universal Time UT by +2 hours. An example: The Moon will leave totality at 03 hours 29.4 minutes UT. This is equal to 05 hours 29.4 minutes Central European Summer Time.
Note also that the planet Saturn will be visible near the eclipsed Moon!
Here is where this eclipse will be visible (quoting from the Astronomical Almanac):
The beginning of the eclipse: USA and Canada, most of Mexico, Central America, South America, Greenland, Europe, Africa, Western Asia, parts of Antarctica, the eastern South Pacific Ocean, the Atlantic Ocean, and the western half of the Indian Ocean.
The end of the eclipse: North America (except parts of Alaska), Hawaii, Central America, South America, the western half of Africa, Europe (except for the extreme east), Greenland, parts of Antarctica, the eastern Pacific Ocean, and the Atlantic Ocean.
As you can see, Lunar Eclipses are global events which can be observed from many geographical regions on the Earth. Contrarily, Total and Partial Solar Eclipses can only be seen from smaller regions.
The scientific belief that the Earth is a (round) sphere is more than 2500 years old. Scientists like the well-known Pythagoras (who lived at approximately 580 BC) concluded this directly by observing Lunar Eclipses. He observed the circular Earth shadow wander across the Moon, exactly as you will when you watch the eclipse on September 27. The only object, which always gives a circular shadow, is a 3-dimensional sphere. Thus, the Earth must be round!
Later, scientists like Eratosthenes, Hipparchos, Ptolemy, Plinius all remarked that eclipses are a powerful tool when mapping the Earth.
In Geography and Navigation, you need a parameter called the Geographical Longitude:
This Longitude is an often applied east-west division of our globe. For more than 1000 years, eclipses were the only reliable method to determine this longitude.
Plinius the Elder (AD 24-79) was an Admiral in the Roman Navy, and he had a strong interest in Science. In fact, he died because he was too close to the volcano Vesuvius during his investigations of the terrible eruptions that destroyed the city of Pompeii in AD 79.
Plinius wrote a famous book on General Science. In this he mentions one lunar eclipse in particular, that took place at the time of the battle of Arbela; this location is in present-day Iraq. During this battle, the moon was eclipsed, two hours after sunset. However, in Sicily, further to the west, the eclipse happened exactly at sunset.
This time difference of 2 hours thus tells that the distance between Arbela and Sicily is two-twentyfourths (2 hours divided by 24 hours) of the circumference of the Earth. In our example above, Sicily and Arbela must then have a difference of longitude of 2/24 of 360 degrees = 30 degrees.
Based on such measurements, Ptolemeus (140 AD) was able to publish quite good maps, showing both (East-West) Longitude and (North-South) Latitude.
Plinius also mentions that another eclipse was observed, both from Italy and Armenia. The distance between these countries was known to be about 3500 kms (in present day units). The eclipse in Armenia happened - according to Plinius - nearly 4 hours later than the Italian eclipse.
Calculate the Earth's circumference (the length of the equator) l and radius r from these observations ( l = Pi x 2 x r).
How well do these figures compare with the correct ones?
Unfortunately, measurements like those above made Ptolemy adopt an Earth circumference and radius that were about 30 percent too small. And this reduced size of the Earth was accepted without scepticism for more than 1000 years.
This finding of Ptolemy also did not leave much space for additional continents. The 1492 globe below, made the same year Columbus first sailed westwards in search of a new route to these countries, shows how India, Japan and China occupy the region that is now reserved for North and South America.
Columbus did not know he had arrived at a new continent, then unknown in Europe. Until his death he firmly believed that the Islands on which he landed belonged to `West'-India.
A Lunar Eclipse that occured during his second voyage on September 14, 1494, could have shown Columbus that he had actually discovered a new continent. But unfortunately, he made a number of simple calculation errors. For more information, see for instance the student project article published in the September 1996 issue of the popular astronomy journal Sky & Telescope.
Another navigator-astronomer, Americo de Vespucci, made more careful observations during a voyage to the `New World' in 1499. He discovered that `West-India' was part of an entirely new continent, and this is one of the main reasons that this continent is now called America, and not Columbia.
Imagine that the moment of the start of totality of our eclipse is observed from both Lisbon (Portugal) and Havana (Cuba). That is, when the last spot on the Moon that is directly illuminated by the Sun, suddenly disappears.
In Lisbon, the total eclipse begins 14 hours 28 minutes after Local Noon (`Local Noon' is when the Sun is seen in the direction of South).
In Havana, the total eclipse begins approximately 9 hours 35 minutes after Local Noon.
Calculate the difference in geographical longitude (in degrees) between these two cities, and compare with your map.
We suggest that you and your group participate actively by observing this Lunar Eclipse. Your subsequent report will be most welcome and should be submitted as indicated below.
Here are few remarks about these observations which may be useful for you and your group.
First you should note that the eclipsed Moon will not be completely dark; you will be able to see it all the time during the eclipse. Its colour will be dark red like it is shown on the figure above. This strange colour is the result of sunlight being refracted when it passes through the Earth's upper atmosphere. Some of this light reaches the Moon. This is why the Moon is not completely dark, even when it is 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.
Note your geographical position (longitude and latitude) (you may find them on a map of your country)
Please try to observe the accurate time when the totality begins (and, if possible, when it ends). If you use an ordinary binocular/telescope, you will have no doubt when the totality begins. It is the time the last, bright spot on the Moon disappears.
Moreover, if you have the time for doing daytime observations too, you should complement your work with a measurement of the time of Local Noon at your position. You will find here a full description of `Local Noon', what it means and how to measure it. However, if you are unable to do these daytime measurements, never mind, the Astronomy On-Line organisers will still tell you what the time of Local Noon would have been at your location, so this should not keep you from reporting your observations!
Finally, send a short report by email with your Group Designation, the timing(s) and your position. By clicking on the line below you get the mailserver. Then write the information about your observations in the space reserved for the message.
All observations of the Total Lunar Eclipse that are received from Astronomy On-Line Groups will be compared by the EAAE European Student Project Group. Some days later, they will be published in a joint article with tables in the Astronomy On-Line Newspaper, together with an evaluation of the results. Remember, your Group and your results will also be mentioned here, if you send in the measurements.
Thereafter you will be able to compute your geographical longitude by comparing your timing(s) with those of other groups, exactly as in the examples above. But now you have real, up-to-date data at your disposal. How close will you come to the correct longitude?
Have a nice time!
Addition (October 7, 1996): The first results of the observations by Astronomy On-Line Groups are now available!
This project was prepared by the European Student Project Group of the European Association for Astronomy Education (EAAE):
| Josée Sert
(France)
Francis Berthomieu (France) Brian Stockwell (UK) Anders Västerberg (Sweden) Mogens Winther (Denmark) |
The text here does not tell you everything about lunar eclipses, but if you want to know more, we suggest that you consult the Sky & Telescope Eclipse Pages which contain detailed predictions for the timing, advice about photographic work, etc. You will find very interesting information about other eclipses here as well, for instance about the total solar eclipse which can be seen in Europe on August 11, 1999. You may also look at the Sky & Telescope issues of September 1996 (page 68-71) or October 1992 (page 437).
More related information is also available in the Astronomy On-Line Solar Eclipse Project and you may find it interesting to read about the ancient observations in a most fascinating science book: `Science for the Citizen' by Lancelot Hogbein (Aberdeen, Sept. 1939).