Tag Archives: astronomy

A lunation

In astronomy, new moon is the first phase of the Moon, when it orbits as seen from the Earth, the moment when the Moon and the Sun have the same ecliptical longitude. [1] The Moon is not visible[note 1] at this time except when it is seen in silhouette during a solar eclipse when it is illuminated by earthshine. See the article on phases of the Moon for further details.

A lunation or synodic month is the mean (average) time for from one new moon to the next. In the J2000.0 epoch, the average length of a lunation is 29.530588 days (or 29 days, 12 hours, 44 minutes, and 3 seconds). However, the length of any one synodic month can vary from 29.26 to 29.80 days due to the perturbing effects of the Sun’s gravity on the Moon’s eccentric orbit.[2] In a lunar calendar, each month corresponds to a lunation.


A short guide to celestial navigation

The CD-Sextant
Build your own sextant

One of the obstacles to learn and practice celestial navigation is the price and availability of sextants. Even the simplest plastic models cost between US$ 50 and 150 and can only be found in a few specialized stores. This scares interested people away from celestial navigation. The X-tant Project , a “do-it-yourself” octant design.  while cheap to build, it requires electric tools, some hard to find materials and considerable work.

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The Making of Planet Earth – National Geographic Full Documentary

Published on Jun 19, 2013

❶ HD Documentary Channel: For all your Space, Universe, Science and Technology documentaries!

Published on Oct 4, 2013
This edition of COSMIC JOURNEYS explores the still unfolding story of Earth’s past and the light it sheds on the science of climate change today. While that story can tell us about the mechanisms that can shape our climate. it’s still the unique conditions of our time that will determine sea levels, ice coverage, and temperatures.

Ice, in its varied forms, covers as much as 16% of Earth’s surface, including 33% of land areas at the height of the northern winter. Glaciers, sea ice, permafrost, ice sheets and snow play an important role in Earth’s climate. They reflect energy back to space, shape ocean currents, and spawn weather patterns.

But there are signs that Earth’s great stores of ice are beginning to melt. To find out where Earth might be headed, scientists are drilling down into the ice, and scouring ancient sea beds, for evidence of past climate change. What are they learning about the fate of our planet… a thousand years into the future and even beyond?

30,000 years ago, Earth began a relentless descent into winter. Glaciers pushed into what were temperate zones. Ice spread beyond polar seas. New layers of ice accumulated on the vast frozen plateau of Greenland. At three kilometers thick, Greenland’s ice sheet is a monumental formation built over successive ice ages and millions of years. It’s so heavy that it has pushed much of the island down below sea level. And yet, today, scientists have begun to wonder how resilient this ice sheet really is.

Average global temperatures have risen about one degree Celsius since the industrial revolution. They could go up another degree by the end of this century. If Greenland’s ice sheet were to melt, sea levels would rise by over seven meters. That would destroy or threaten the homes and livelihoods of up to a quarter of the world’s population.

With so much at stake, scientists are monitoring Earth’s frozen zones… with satellites, radar flights, and expeditions to drill deep into ice sheets. And they are reconstructing past climates, looking for clues to where Earth might now be headed… not just centuries, but thousands of years in the future.

Periods of melting and freezing, it turns out, are central events in our planet’s history.
That’s been born out by evidence ranging from geological traces of past sea levels… the distribution of fossils… chemical traces that correspond to ocean temperatures, and more.

Going back over two billion years, earth has experienced five major glacial or ice ages. The first, called the Huronian, has been linked to the rise of photosynthesis in primitive organisms. They began to take in carbon dioxide, an important greenhouse gas. That decreased the amount of solar energy trapped by the atmosphere, sending Earth into a deep freeze.

The second major ice age began 580 million years ago. It was so severe, it’s often referred to as “snowball earth.” The Andean-Saharan and the Karoo ice ages began 460 and 360 million years ago. Finally, there’s the Quaternary… from 2.6 million years ago to the present. Periods of cooling and warming have been spurred by a range of interlocking factors: the movement of continents, patterns of ocean circulation, volcanic events, the evolution of plants and animals.

The world as we know it was beginning to take shape in the period from 90 to 50 million years ago. The continents were moving toward their present positions. The Americas separated from Europe and Africa. India headed toward a merger with Asia. The world was getting warmer. Temperatures spiked roughly 55 million years ago, going up about 5 degrees Celsius in just a few thousand years. CO2 levels rose to about 1000 parts per million compared to 280 in pre-industrial times, and 390 today.

But the stage was set for a major cool down. The configuration of landmasses had cut the Arctic off from the wider oceans. That allowed a layer of fresh water to settle over it, and a sea plant called Azolla to spread widely. In a year, it can soak up as much as 6 tons of CO2 per acre. Plowing into Asia, the Indian subcontinent caused the mighty Himalayan Mountains to rise up. In a process called weathering, rainfall interacting with exposed rock began to draw more CO2 from the atmosphere… washing it into the sea. Temperatures steadily dropped.

By around 33 million years ago, South America had separated from Antarctica. Currents swirling around the continent isolated it from warm waters to the north. An ice sheet formed. In time, with temperatures and CO2 levels continuing to fall, the door was open for a more subtle climate driver. It was first described by the 19th century Serbian scientist, Milutin Milankovic.

He saw that periodic variations in Earth’s rotational motion altered the amount of solar radiation striking the poles. In combination, every 100,000 years or so, these variations have sent earth into a period of cool temperatures and spreading ice.

Μηχανισμός των Αντικυθήρων

More than 21 centuries ago, a mechanism of fabulous ingenuity was created in Greece, a device capable of indicating exactly how the sky would look for decades to come — the position of the moon and sun, lunar phases and even eclipses. But this incredible invention would be drowned in the sea and its secret forgotten for two thousand years.

This video is a tribute from Swiss clock-maker Hublot and film-maker Philippe Nicolet to this device, known as the Antikythera Mechanism, or the world’s “first computer”. The fragments of the Mechanism were discovered in 1901 by sponge divers near the island of Antikythera. It is kept since then at the National Archaeological Museum in Athens, Greece.

For more than a century, researchers were trying to understand its functions. Since 2005, a pluridisciplinary research team, the “Antikythera Mechanism Research Project”, is studying the Mechanism with the latest high tech available.

The results of this ongoing research has enabled the construction of many models. Amongst them, the unique mechanism of a watch, designed by Hublot as a tribute to the Mechanism, is incorporating the known functions of this mysterious and fascinating ancient Mechanism.

A model of the Antikythera Mechanism, built by the Aristotle University in Greece, together with the mechanism of the watch and this film in 3D are featuring in an exhibition about the Mechanism that is taking place in Paris, at the Musée des Arts et Métiers.

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The Gregorian calendar

The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely accepted civil calendar.[1][2][3] It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582; the decree, apapal bull, is known by its opening wordsInter gravissimas.[4] The Gregorian calendar was adopted initially by the Catholic countries of Europe, with other countries adopting it over the following centuries.

The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost 11 minutes shorter. The discrepancy results in a drift of about three days every 400 years. At the time of Gregory’s reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable.

The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory’s time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by the Calabrian doctorAloysius Lilius (or Lilio).[5] Lilius’s proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati. Lilio also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.

The Gregorian calendar thus modified the Julian calendar’s regular cycle of leap years as follows:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.[6]

In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. Between AD 325 (when the First Council of Nicaea was held, and the vernal equinox occurred approximately 21 March), and the time of Pope Gregory’s bull in 1582, the vernal equinox had moved backward in the calendar, until it was occurring on about 11 March, 10 days earlier. The Gregorian calendar therefore began by skipping 10 calendar days, to restore March 21 as the date of the vernal equinox.

Because Protestants and Eastern Orthodox Christians did not recognize the authority of the Pope, many European countries did not initially follow the Gregorian reform, and maintained their old-style systems. Eventually other countries followed the reform for the sake of consistency, but by the time the last adherents of the Julian calendar in Eastern Europe (Russia and Greece) changed to the Gregorian system in the 20th century, they had to drop 13 days from their calendars, due to the additional difference between the two calendars accumulated after 1582.

The Gregorian calendar continued the previous year-numbering system (Anno Domini), which counts years from the traditional date of the nativity, originally calculated in the 6th century and in use in much of Europe by the High Middle Ages. This year-numbering system, now also called Common Era, is the predominant international standard today

NASA | Computer Model Shows a Disk Galaxy’s Life History

Published on Oct 19, 2012
This cosmological simulation follows the development of a single disk galaxy over about 13.5 billion years, from shortly after the Big Bang to the present time. Colors indicate old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years across. The simulation ran on the Pleiades supercomputer at NASA’s Ames Research Center in Moffett Field, Calif., and required about 1 million CPU hours. It assumes a universe dominated by dark energy and dark matter. Credit: F. Governato and T. Quinn (Univ. of Washington), A. Brooks (Univ. of Wisconsin, Madison), and J. Wadsley (McMaster Univ.).

This video is public domain and can be downloaded here:

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