Category Archives: Astronomy

How To Celebrate Each Christmas Multiple Times

Commerce has tricked many people into believing that Christmas begins with Black Friday sales.

Of course, nothing could be further from the truth. The four weeks before Christmas are called “Advent”. Thereafter, Christmas begins with Holy Night (the night from December 24th to December 25th). Or does it?

Well, if you really love Christmas, you may actually enjoy it three (or even four) times. How come?

First of all, there are no historical records of the birth date of Jesus of Nazareth. So the chosen date for celebrating the birth of Jesus Christ was picked rather arbitrarily. However, it stands to reason that the date was chosen to coincide (roughly) with winter solstice. This natural event had been a sacred day ever since human civilization grasped the rudimentary fundamentals of celestial cycles. The Romans (as well as many other literate cultures) had of course more than just a basic understanding of this.

By the way, contrary to popular belief, Christmas is not the most important holiday in Christianity, nor was it even celebrated in the early beginnings. 

In 330 AD, the Roman Empire split into two parts: the western half centered in Rome, and the eastern half centered in Constantinople. The churches of the Western Roman Empire continued to celebrate Christmas as a minor holiday on December 25.

Mosaic of Jesus as Christus Sol (Christ the Sun) in Mausoleum M in the pre-fourth-century necropolis under St Peter’s Basilica in Rome.

But in the East, the birth of Jesus began to be celebrated in connection with the Epiphany, on January 6. This holiday was not primarily about the the birth of Jesus, but rather his baptism. The feast was introduced in Constantinople in 379, in Antioch towards the end of the fourth century ( probably in 388) and in Alexandria in the following century.

When it comes to marking days, all conventional, numerical calendar systems suffer from an astronomical problem: During the time it takes for the Earth to complete one full orbit around the Sun, our planet rotates 365.256 times around its own axis. What this means is that the day isn’t really 24 hours long. Earth spins once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to other, distant, stars.

So one Earth year isn’t 365 days long, but precisely 365 days, 5 hours, 48 minutes and 46 seconds. How does one put that into a calendar when we define each 24-hour period as a “day”? And when we divide the year into 12 months – which, by the way, are all arbitrary, man-made definitions? There really are are only four natural demarkations in the year that play a major role in our lives: the two equinoxes and the two solstices, which mark the beginnings of each season.

For completeness, because Earth’s orbit isn’t a circle but an ellipsis, there are the two apsides, the two extreme points of Earth distance to the Sun: Aphelion (apoapsis) and perihelion (periapsis). But these two would have little to no practical effect except for those studying such things.

The Julian Calendar, was proposed by Julius Caesar in 46 BC and enacted by edict on January 1, 45 BC. No, Julius did’t invent it. It was a reform of an earlier Roman calendar and probably designed by Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.

Whoever did it, the Julian Calendar was undoubtedly one of mankind’s most remarkable intellectual accomplishments. But as all human explanations of natural phenomena, it is rather imperfect in that although its synchronization with the solar year is better than most other calendars, the deviations get bigger and bigger as time goes on. Eventually, the year’s four seasons and the calendar will become way off.

One can correct these deviations from the natural world by throwing in “leap time” – a strange concept in which we are basically making up days, months or even years. (In other words, we are violating the rules of science by purposely making our interpretation of the reality fit the observation).

In order to reduce the need for such calendar doctoring, Pope Gregory XIII in October 1582 introduced the Gregorian calendar as a correction of the Julian calendar.

Lunario Novo, Secondo la Nuova Riforma della Correttione del l’Anno Riformato da N.S. Gregorio XIII, printed in Rome by Vincenzo Accolti in 1582, one of the first printed editions of the new calendar.

This has liturgical significance since calculation of the date of Easter assumes that spring equinox in the Northern Hemisphere occurs on March. To correct the accumulated error, Pope Gregory ordained the date be advanced by ten days. (One can do that is one is pope).

Pope Gregory XIII, portrait by Lavinia Fontana, 16th Century.

Most Roman Catholic lands adopted the new calendar immediately. (Not that they had much of a choice). The clerical leaders of Protestant lands who did not recognize the pope’s authority of course protested. But eventually, they too ended up following suit over the following 200 years. (Not because Protestants admitted the pope’s new calendar was a good innovation, but because having two different calendars caused quite a bit of confusion. Let’s just say it wasn’t popular with the masses).

The British Empire (including the American colonies) adopted the Gregorian Calendar in 1752 with the Calendar (New Style) Act 1750. At that time, the divergence between the two systems had grown to eleven days.

This meant that Christmas Day on December 25 (“New Style”) was eleven days earlier than it would have been but for the Act, making “Old Christmas” (“Old Style”) on December 25 happen on January 5 (“New Style”).

In February 1800, the Julian calendar had yet another leap year but the Gregorian calendar did not. This moved Old Christmas to 6 January (“New Style”), which coincided with the Feast of the Epiphany.

For this reason, in some parts of the world, the Feast of the Epiphany, which is traditionally observed on 6 January, is sometimes referred to as “Old Christmas” or “Old Christmas Day”.

So where do we stand today? According to the Gregorian calendar, Western Christianity and part of the Eastern churches celebrate Christmas on December 25.

The Armenian Apostolic Church, the Armenian Evangelical Church, and some Anabaptists (such as the Amish people) still celebrate “Old Christmas Day” on January 6.

Meanwhile, most Oriental Orthodox and part of the Eastern Orthodox churches celebrate on January 7 (which corresponds to “Old Style” December 25).

Lastly, the Armenian Patriarchate of Jerusalem maintains the traditional Armenian custom of celebrating the birth of Christ on the same day as Theophany (January 6), but it uses the Julian calendar for the determination of that date. As a result, this church celebrates “Christmas” (actually, Theophany) on what the majority of the world now considers to be January 19 on its Gregorian calendar.

Russian icon of the Theophany (the baptism of Jesus by John the Baptist) (6 January), the highest-ranked feast which occurs on the fixed cycle of the Eastern Orthodox liturgical calendar.

So there you have it! The dates of Christmas are neither prescribed by God nor nature, but by man. They are nothing but human convention, and they differ because of the problems inherent in making a calendar that accurately reflects nature, along with some religious differences. Celebrate as you wish!

Merry Christmas.

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Halley's Comet May 29, 1910

110 Years Ago: Earth’s Passage Through Comet Halley’s Tail Mesmerizes The Public

On May 19 / May 20, 1910, Earth passed through the tail of Halley’s Comet with great fanfare.

The event is meaningful to me for two reasons. First, my grandmother told me about it when I was little. She herself was a little girl in 1910, and her memories were not very detailed. But she recalled, as her strongest memory, the general feeling of excitement among the adults around her. Some must have been genuinely panicked, others were probably nervous, and yet others were mocking those who suffered from vivid superstitions.

From German: “Old woman, close the umbrella. When the comet sees you, it’ll tun around and Earth is saved.”

Today, few people know that there was actually another comet visible in the sky earlier that year of 1910. The “Great January Comet of 1910”, officially designated “C/1910 A1” was a surprise visitor in the sky. Already visible to the naked eye when it was first reported on January 12, it brightened very suddenly, to the point where it eventually became brighter than Venus, and was visible during the day.

First spotted in the southern hemisphere, it reached perihelion on January 17 with a magnitude of –5. It then declined in brightness but became a spectacular sight from the northern hemisphere in the evening twilight. By early February, its curved tail reached 50 degrees into the sky.

There were of course plenty of newspaper accounts. The public, not yet accustomed to front page astronomical news, became highly interested in comets, and in what the experts had to say — especially at a time when superstitions and the belief in metaphysics was much more widespread than today.

At the time, Halley’s Comet, which had been known since ancient times, had been calculated to reach its perihelion on April 20, based on Newtonian physics and the work of Edmond Halley.

Illustration from the January 1910 issue of Popular Science Monthly magazine, showing how Halley’s tail points away from the Sun as it passes through the inner Solar System

Astrophotography and astrospectography were new fields, they were used to detect toxic gas cyanogen gas in the comet’s tail. The highly famous French astronomer and author Nicolas Camille Flammarion speculated that, when Earth passed through the tail, the poison gas “would impregnate the atmosphere and possibly snuff out all life on the planet.”

Flammarion was not only a genius scientist and author, but also a man with rather esoteric beliefs. He believed not only in the transmigration of souls, but also in telepathy, apparitions, hauntings, and “psychic forces”.

Very quickly, all manner of profiteers, charlatans, mystics, and those purporting to possess special astrological insights, seized on the opportunity, and soon, the panicked public was buying up quack “anti-comet pills”, “anti-comet umbrellas” and gas masks. Sadly, we even find newspaper accounts of people committing suicide because they didn’t want to see the catastrophe.

Considering the nature of what left the strongest impression in my grandmother’s memories, I wonder what today’s small children will remember, many decades from now, about the current COVID-19 crisis. Surely, it will be memories about how we adults reacted, which should also give us reason for contemplation.

The other reason why Halley’s Comet interests me is its association with one of my favorite authors and personalities. Mark Twain was born November 30, 1835, exactly two weeks after the comet’s previous perihelion. In his autobiography of 1908, he writes:

I came in with Halley’s comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don’t go out with Halley’s comet. The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’

Twain died on 21 April 1910, the day following the comet’s subsequent perihelion. This is how the comet looked that day:

Portion of Plate b41215 of Halley’s comet taken on April 21, 1910 from Arequipa, Peru with the 8-inch Bache Doublet, Voigtlander. The exposure was 30 minutes centered on 23h41m29s R.A. and +07d21m09s Declination.

– 30 –

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Light Pollution

Light Pollution

Photo: NASA and NOAA. Click to enlarge.

The natural night skies as seen from Earth are awe inspiring. But ever since the invention of electric lighting, unobstructed dark skies have been disappearing from industrialized, populated areas. Sadly, most people living in the white areas of the picture above have never had a chance to experience the firmament’s full glory.

More information about light pollution and the importance of fighting it:

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Mars, Four Billion Years Ago

Four billion years ago, Earth was a rather hellish place. Its crust was still very thin and unstable. There was heavy and violent volcanic activity. Earthquakes were shaking the ground, liquid lava flowed in many places, and poisonous gases were everywhere. On top of that, our poor little planet was bombarded by a constant barrage of large meteorites. Despite all of that, the first primitive life forms are thought to have appeared during this period.

By contrast, at the same time, Mars was a very tranquil place. Here is a NASA animation, based on the latest data we have, showing what the surface of Mars would likely have looked like at the same time. Blue skies, clouds, oceans, lakes, rivers, mountains and all. Almost like a place for a vacation resort!

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Earth As Seen From Saturn

This picture shows us. All of us on planet Earth. All 7 billion human beings contained one tiny dot of light.

Earth from Saturn

(Click to enlarge)

In this rare image taken on July 19, 2013, the wide-angle camera on NASA’s Cassini spacecraft has captured Saturn’s rings and our planet Earth and its moon in the same frame. It is only one footprint in a mosaic of 33 footprints covering the entire Saturn ring system (including Saturn itself).  At each footprint, images were taken in different spectral filters for a total of 323 images: some were taken for scientific purposes and some to produce a natural color mosaic.  This is the only wide-angle footprint that has the Earth-moon system in it.

The dark side of Saturn, its bright limb, the main rings, the F ring, and the G and E rings are clearly seen; the limb of Saturn and the F ring are overexposed. The “breaks” in the brightness of Saturn’s limb are due to the shadows of the rings on the globe of Saturn, preventing sunlight from shining through the atmosphere in those regions.  The E and G rings have been brightened for better visibility.

Earth, which is 898 million miles (1.44 billion kilometers) away in this image, appears as a blue dot at center right; the moon can be seen as a fainter protrusion off its right side. An arrow indicates their location in the annotated version. (The two are clearly seen as separate objects in the accompanying narrow angle frame: PIA14949.) The other bright dots nearby are stars.

This is only the third time ever that Earth has been imaged from the outer solar system. Continue reading

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Flying Over the Earth at Night

This video is so amazing that I had to repost it as it came. This was originally posted on “Astronomy Picture of the Day“, a daily blog site highly recommended for everyone with an interest in astronomy.

Video Credit: Gateway to Astronaut PhotographyNASA ; Compilation: David Peterson (YouTube);
Music: Freedom Fighters (Two Steps from Hell)

Explanation: Many wonders are visible when flying over the Earth at night. A compilation of such visual spectacles was captured recently from the International Space Station (ISS) and set to rousing music. Passing below are white cloudsorange city lightslightning flashes in thunderstorms, and dark blue seas. On the horizon is the golden haze of Earth’s thin atmosphere, frequently decorated by dancing auroras as the video progresses. The green parts of auroras typically remain below the space station, but the station flies right through the red and purple auroral peaks. Solar panels of the ISS are seen around the frame edges. The ominous wave of approaching brightness at the end of each sequence is just the dawn of the sunlit half of Earth, a dawn that occurs every 90 minutes.


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by Tim Tompson*

Easter Sunday will soon be upon us.

In the year 325 A.D. the First Council of Nicaea defined the date for Easter as the first Sunday following the first Full moon after the Vernal equinox. But astronomically selected dates can move around; e.g., the vernal equinox can happen on 20 March as well as 21 March, and the phases of the moon are not tied either to the civil calendar nor to the equinoxes. So, for the purposes of calculating the date for Easter, the Roman Catholic church defines its own equinox as always happening on 21 March, and they use their own “ecclesiastical full moon”, which by definition always occurs on the 14th day of the ecclesiastical lunar month on the ecclesiastical lunar calendar (which assumes by definition that 19 tropical years equals 235 synodic months exactly [the correct number is 234.997]). In this way, Roman Catholic Easter always falls in the window of 22 March to 25 April.

Roman Catholics use the Gregorian calendar, which was finalized in 1582 for the explicit purpose of returning the date of Easter to the same date it had when the First Council of Nicaea met. In the time between 325 and 1582, the vernal equinox had slipped backwards through the civil calendar to 11 March instead of 21 March, which it was in 325. So when the Gregorian calendar replaced the Julian calendar in 1582, ten days were skipped over, which moved Easter back to 21 March. And by the trick of skipping leap days in years ending in 00, unless they are evenly divisible by 400, the length of the average civil calendar year is shortened from 365.25 days (that is one “tropical year”) to 365.2425 days, the end result of which is that the civil calendar will fall behind the seasons by about one day come the year 3200 (a problem easily solved by skipping the leap day in 3200).

The actual time it takes to go from one vernal equinox to the next is 365.24219878125 days, which should result in an accumulated difference between the seasons and the civil calendar of 3 days, 17 minutes, 33 seconds over 10,000 years. But the mean tropical year is decreasing by 0.53 seconds per 100 years (a slow tidal transfer of energy from sun to Earth), and the mean length of day is decreasing by 0.0015 seconds per 100 years (a slow tidal transfer of energy from Earth to moon). That’s why the calendar can lose a whole day in only about 1200 years. One could cleanup the next few thousand years by skipping the leap day in the year 3200, keep the leap day in 3600 and 4000, and skipping the leap day in 4500 & 5000.

Eastern Christians (mostly the Eastern and Greek Orthodox churches, Eastern Catholic and Coptics) use the old Julian calendar, so they celebrate Easter basically a month later than do the Romans, in the window between 4 April to 8 May.

There are various good reasons for having a civil calendar that is locked to the seasons. But the one that has been most important has proven to be the need to have Easter fall on a fixed time of the civil calendar year.

* The writer is a physicist retired from NASA’s Jet Propulsion Laboratory. Among his main personal interests are astronomy, chess, languages and linguistics, and military history.

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Lowell Observatory: Fundraising To Save Clark Telescope

Percival Lowell observing Venus from the observer's chair of Clark telescope at the Lowell Observatory in 1910.

Percival Lowell observing Venus from the observer’s chair of Clark telescope at the Lowell Observatory in 1910.

Percival Lowell was convinced that there had to be life on Mars, and he spent much of his life trying to prove it. (I often wonder how excited he would be about our Mars rovers).Of course Lowell had no such help. In 1895, the astronomer commissioned a telescope he thought suitable for the visual examination of the surface of Mars. The refracting telescope was made by Alvin Clark & Sons (leading telescope makers of the time) and is housed at Lowell Observatory in Flagstaff. It is among the most important, historical science landmarks.

Today, the 117-year-old “Clark telescope” is in need of major restoration, and the Lowell Observatory is holding a crowdsourcing campaign to raise funds.

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You Are Here

Click to enlarge.

This is the current map of the Milky Way, as best we can make it out from our viewpoint inside. Our home galaxy has about 200 billion to perhaps 500 billion other stars beside our own. Orbiting them are billions to trillions of planets and moons. And that’s just one of billions of galaxies in the universe.

We cannot take pictures of our galaxy, so this image is a model. It is based primarily on results from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and the Multiband Imaging Photometer for Spitzer Galactic Plane Survey (MIPSGAL). Both are key projects for the infrared Spitzer Space Telescope. This particular image was released on June 3, 2008 at the 212th meeting of the American Astronomical Society.

I hope your head is spinning. (It should be).

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Transit Of Venus

Like thousands of other people, I watched the Venus transit on my computer, via webcast from the Keck Observatory on the summit of Mauna Kea on Hawaii. (It was amusing to see a bunch of excited scientists trying to keep a TV audience entertained for  6 hours).

There are plenty of great transit photographs taken by amateurs. (The L.A. Times has a nice collection of images from Southern California). But the best view was from space. My favorite image is this one, taken by the Japanese Hinode spacecraft.

(Click to enlarge).

Hinode is a collaboration between the Japanese space agency JAXA and various institutions in the US and the UK. Then known as “Solar-B”, the satellite was launched on a Japanese M-5 rocket in 2006. Earlier this year, Hinode returned  stunning images and data from the Lunar eclipse. Well done, Japan!


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