Longest river

Longest river

The Nile River of Africa is 4,160 miles, or 6656 km, long -- the longest river in the world. Its waters drain 1/10 of the area of Africa, including parts of present day Egypt, Sudan, Ethiopia, Uganda and Rwanda. The Nile Valley gave life to two of the world's first civilizations, the related cultures of Egypt and Nubia.

The growth of civilization led to a rapid growth of mathematics and science. People have always looked for patterns in the world around them, patterns that they could use. They found a very important pattern in the seasonal changes of the Nile River.

Once a year, the Nile River changes dramatically. For most of the year, the White Nile quietly joins the Blue Nile to form the great Nile River at Khartoum in Sudan. But at the end of August, there is a sudden change. Heavy rains in the highlands of Ethiopia wash floodwaters and silt into the Blue Nile. The Blue Nile swells to twice its usual size and turns the Nile River into a roaring flood. The flood rushes north, plunging over six cataracts.

Nowadays, the floodwaters are held in the artificial Lake Nasser and released year-round to turn electric generators and water crops. But it was different in ancient times. The flood used to rush downhill over 1000 miles, through Nubia and Egypt to the Mediterranean Sea. The land near the river was flooded for weeks. People were glad to see the flood, because it brought water for their crops. When the river went down, it left a rich silt that fertilized the fields. Rain was rare. The desert took over on the high ground where the floodwaters did not reach. For people of the Nile Valley, the timing of the flood was of the greatest importance.

Flood and star
Thousands of years ago, African scientists began to count the number of days between the beginning of one Nile flood and the next. They kept these records for hundreds of years. Their records showed that, on average, there were 365 days between the first appearances of the floods. Astronomers made another exciting discovery. They noticed that the bright star Sirius reappeared in the sky as the Nile floodwaters neared Memphis in Egypt. The same thing happened after 365 days, and again after another 365 days.

The study of star and flood patterns led the Egyptians to design the first 365-day, solar calendar. The official Egyptian calendar had 12 months with 30 days to each month. They added five days for New Year celebrations to make a year of 365 days. A later pharaoh added a leap day every four years to the average year 365 1/4 days long.

Lunar calendar, solar calendar
The Egyptian government still kept the old lunar calendar to mark religious holidays. Lunar calendars are based on the 29.5-day moon cycle. (Even today, people use lunar calendars to find the date of the Christian Easter or the Jewish Passover or Muslim Ramadan.) Unfortunately, lunar calendars were very complicated; 12 months of 29 days made a year that was 17 days short of 365. To make up for short 12-month years, some lunar years had to have 13 months.

When Julius Caesar came to Egypt to visit Cleopatra, he became acquainted with the 365-day Egyptian calendar. It was the only calendar that had the same number of months each year. Julius Caesar took the 365-day calendar back with him to Europe where it became known as the Julian calendar. Its Egyptian origin was forgotten.

The ancient Egyptians were also first to divide the day into 24 hours, 12 hours of night and 12 hours of day. At first the hours of day and night varied with the season, but later the Egyptians made all the hours of equal length.

This activity was adapted from Algebra Activities from Many Cultures by Beatrice Lumpkin. Available from:

J. Weston Walch, Publisher 
P.O. Box 658 
Portland, Maine 04104-065