Early European astronomers determined Easter dates

Apr 25, 2011
In this Sun calendar, a hole in the ceiling of the cathedral projects a shaft of sunlight onto this bronze strip on the pavement below which is engraved with the days of the year and signs of the zodiac. The Basilica of San Petronio is in Bologna, Italy. Credit: Calter Photo

How do they know it’s Easter? Ever wondered how the exact dates of the Easter break are chosen? Easter Sunday can fall anytime between 22 March and 25 April and, thanks to European observations of the Sun that go back many centuries, the exact date can be predicted as far ahead as 4099 AD.

Back in 325 AD, it was declared that should be celebrated on the first Sunday after the first full Moon following the vernal equinox (the Spring day in the northern hemisphere when the hours of daylight and darkness are equal). Over the next few centuries, theologians and scientists struggled with the problem of calculating these vital dates years in advance. Although they often studied the skies in some detail to help them work out future calendars, this was particularly difficult when working on the premise that the Sun moved around the Earth. Then, in 1651, Giovanni Cassini installed a pinhole camera in the roof of the San Petronio cathedral in Bologna in Italy.

Previous studies of the movement of the Sun indicated the urgent need for calendar reform in the sixteenth century and the introduction of the new Gregorian calendar, that is still in use today. But it was Cassini who installed the most accurate observatory at San Petronio, and made ample use of it to monitor the accuracy of the new calendar. Cassini’s observations allowed exact calculations of future equinoxes on the Gregorian calendar to be made in advance, thus helping to solve the ‘When’s Easter?’ problem.

Cassini also used his observations of the Sun’s image projected on to the cathedral floor to try to confirm the theories of Johannes Kepler, the German astronomer who proposed in 1609 that the planets moved in elliptical orbits. By watching the Sun’s projected disc on the cathedral floor shrink and expand as it travelled across the stone, Cassini was able to deduce that the distance between the Sun and Earth did not remain the same, proving that a circular orbit was out of the question. After much trial and error, and thousands of observations, Cassini was able to prove that Kepler’s theories were correct and became the first person to make a reasonably accurate calculation of the distance between the Sun and the Earth. This essentially demolished what was left of classical cosmology, opening the way for modern celestial mechanics.

Cassini discovered the Saturnian satellites Lapetus in 1671, Rhea in 1672, and both Tethys and Dione in 1684. In 1675 he discovered what is now known today as the Cassini Division - Saturn's rings are split largely into two parts by a narrow gap and that the rings were in fact swarms of tiny moonlets too small to be seen individually. Credit: Observatoire de Paris

The exact movement of the bodies in the has fascinated European scientists for centuries. Cassini and Kepler were building on a European astronomical tradition, and their theories were preceded by those of Nicolaus Copernicus, who upset everybody in 1543 when he placed the Sun near the centre of the Universe, with planets in orbit around it.

Nowadays, working out the exact movements of planets and other objects in the Solar System, a discipline known as orbital mechanics, is done using complex mathematics, detailed measurements and computer modelling. Orbital mechanics takes on vital importance when we are sending spacecraft to study objects in the Solar System at close range.

For example, to reach Saturn's moon Titan in 2005, Cassini/Huygens employed a concept called ‘gravity assists’, in other words it uses the gravity of other planets, namely Venus, Earth and Jupiter, in order to bend its trajectory and boost its speed as it flies by these bodies.

Before the idea of gravity assists was proposed in the early 1960s, it would have taken decades to get to Saturn. Using gravity assists, Cassini-Huygens only needed enough energy to get to Venus. After that, the ‘gravitational slingshots’ are all that is required, and the journey will instead take around seven years. Naturally, such a complex trajectory is only made possible using the most precise orbital mechanics.

The tools used may have changed, and accuracy improved beyond measure, but the urgent desire of European to predict accurately the movements of celestial bodies has altered little since the days when Cassini sat on a cold stone floor watching sunbeams.

Explore further: NASA's space station Robonaut finally getting legs

add to favorites email to friend print save as pdf

Related Stories

Rings on the horizon

Jan 26, 2011

The Cassini spacecraft has taken a some recent images of two of Saturn’s most notorious moons, where in both images the planet’s rings serve as a backdrop. Above, Enceladus stands out with its cratered ...

Moon Illusion tricks the eye

Jan 19, 2011

We’ve all experienced the Moon Illusion, where our own full Moon looks bigger when seen on the Earth’s horizon. But how about this illusion where you can’t really tell which of these two moons ...

Saturn images to be displayed in New York

Apr 21, 2008

The National Aeronautics and Space Administration says some of the best images from Saturn and the planet's rings and moons will be displayed in New York.

SDO's crazy-looking Sun due to syzygy

Apr 04, 2011

It looks like something is eating the Sun in recent pictures from the Solar Dynamics Observatory — and in recent SDO videos, the Sun suddenly disappears! What is going on? Could it be aliens, Planet X, ...

Recommended for you

NASA's space station Robonaut finally getting legs

10 hours ago

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Sun emits a mid-level solar flare

Apr 18, 2014

The sun emitted a mid-level solar flare, peaking at 9:03 a.m. EDT on April 18, 2014, and NASA's Solar Dynamics Observatory captured images of the event. Solar flares are powerful bursts of radiation. Harmful ...

Impact glass stores biodata for millions of years

Apr 18, 2014

(Phys.org) —Bits of plant life encapsulated in molten glass by asteroid and comet impacts millions of years ago give geologists information about climate and life forms on the ancient Earth. Scientists ...

The importance of plumes

Apr 18, 2014

The Hubble Space Telescope is famous for finding black holes. It can pick out thousands of galaxies in a patch of sky the size of a thumbprint. The most powerful space telescope ever built, the Hubble provided ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

frajo
5 / 5 (1) Apr 25, 2011
Cassini discovered the Saturnian satellites Lapetus in 1671, Rhea in 1672, and both Tethys and Dione in 1684.
No, not "Lapetus", but Japetus (or Iapetus) - named after one of the Titans of Greek mythology, son of Uranus and Gaia.
frajo
not rated yet Apr 25, 2011
Back in 325 AD, it was declared that Easter should be celebrated on the first Sunday after the first full Moon following the vernal equinox (the Spring day in the northern hemisphere when the hours of daylight and darkness are equal).
This decision of the First Council of Nicaea in 325 holds only for Western Christianity. Eastern and Oriental Orthodox churches still calculate their Easter dates according to the Julian calendar.
Thus, while this year (2011) the central European Easter happens to coincide with Greek Easter, in 2008 there was a five week difference.

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Cosmologists weigh cosmic filaments and voids

(Phys.org) —Cosmologists have established that much of the stuff of the universe is made of dark matter, a mysterious, invisible substance that can't be directly detected but which exerts a gravitational ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...

Airbnb rental site raises $450 mn

Online lodging listings website Airbnb inked a $450 million funding deal with investors led by TPG, a source close to the matter said Friday.