Stargazing: Oh, star of wonder 

Torbay Weekly

Stargazing with John Stapleton of Torbay Astronomical Society:

I suspect that at this time of year the only star that people think about is the one at the top of their Christmas tree.

But have you ever stopped to think why we put a star on the tree at all?

The Biblical story of the birth of Jesus tells us that: “Wise men from the East followed a wondrous star.”

These wise men were clearly astronomers, among other things, and probably came from places such as Chaldea and Babylon to the east of Biblical Judea.

Both places were centres of learning and sources of ancient astronomical texts on clay tablets discovered during archaeological excavations.

We are also told that the star 'appeared to them' suggesting that it was not something they regularly observed or were aware of.

So what could it have been?

The most enduring suggestion has been that the 'star' was actually a great conjunction of two or more planets. This is where the planets appear so close together that it is difficult to separate them individually.

One such conjunction has been calculated to within 100 years or so of the Biblical event.

However, these 'wise men' would surely have been aware of the movements of the planets, and, in fact, the evidence on the clay tablets supports that such observations were routinely made in the region.

Another theory is that the 'star' may have been a comet.

Comets were very significant in ancient times, being seen as portents of doom, and were observed extensively.

There is a story of two Chinese astronomers who failed to predict a comet and were put to death by a horrified Emperor.

So again, experienced observers would surely not have mistaken a comet for anything else although we do know that not all comets appear typical and easily recognisable with a tail trailing behind them.

Halley’s Comet actually appeared in the night sky three times within 100 years either side of the supposed date of the birth of Jesus.

More likely is the occurrence of a much rarer astronomical event: a supernova.

Only eight or nine supernovae bright enough to be visible to the naked eye have been recorded throughout human history.

A supernova occurs when a distant star, sometimes in another galaxy, comes to the end of its life and catastrophically explodes becoming dozens of times brighter than it normally would be for a brief period of time, up to a few weeks.

Unfortunately, none of the supernovae known, and to have their eruption dates calculated, or observed, occurred within 1,000 years of the Biblical date.

The brightest of all these supernovae occurred in the year 1006 and became much brighter than the planet Venus at magnitude -7.5, the next brightest occurred in the year 1054 and reached a magnitude of -6, also brighter than Venus.

The remnant of this supernova is now known as the Crab Nebula and is a good target for telescopes in winter skies.

Both supernovae were observed and recorded by Chinese and Korean astronomers and are the nearest to the Biblical time that we are aware of.

The nearest such supernova prior to the birth of Jesus and of naked eye visibility, occurred in about 4000 BC.

We may never know the source or explanation of the Biblical 'star', it may be marked by some remnant, as yet unidentified, but this is unlikely; it may be purely allegorical, a story device, or it may have been a miracle after all.

So here’s wishing all Torbay Weekly readers a Merry Christmas.

The Star Chart

The sky will look like the chart on December 7 at 9pm and again on December 21 at 8pm. And four minutes earlier on each successive night e.g. 8.56 on December 8.

To use the chart hold it above your head while facing south so that you can look directly from the chart to the sky.

Skynotes

Please note all times given in this article are in GMT and as the clocks have changed that is the current time.

Sun: Throughout December the Sun sets before 4.30pm and there are just eight hours of daylight. The Winter Solstice (shortest day) occurs on December 21. On this date the Earth is closer to the Sun than at any other time in its orbit. We experience winter in the Northern Hemisphere because the North Pole is tilted away from the Sun and its warming energy is spread over a greater area.

Mercury: Mercury is visible in the evenings, setting soon after the Sun. The best time to see Mercury will be on December 31 when it will be close to the much brighter Venus in the sky.

Venus: Venus is an evening object throughout the month, setting about two-and-a-half hours after the Sun. At this time Venus is seen against the background stars of Sagittarius. On December 31, Venus will lie just over 6 degrees (a little over the width of the three middle fingers of your hand at arm’s length) from Mercury and this will be the best time to spot both planets.

Mars: Mars is now a morning object seen against the background stars of Ophiuchus. It is now quite faint and difficult to observe rising two hours before the Sun by the end of the month when it will be near to a narrow crescent moon.

Jupiter: Jupiter now sets shortly after 10pm and will set before 9pm by the end of the month so December is the last chance to get a good view of the giant planet. Jupiter is currently seen against the background stars of Aquarius and can be seen in the southern sky. The Galilean moons can be seen with a good pair of binoculars or a small (bird-spotting) telescope. DSLR images of the planet will also pick up the moons.

Saturn: The ringed planet can be found against the stars of Capricornus low in the south-west. It is now lower in the sky and sets by 7pm at the end of the month, so this is really the last opportunity to view the ringed planet.

Uranus and Neptune: Uranus is visible all night long throughout December and is seen against the background stars of Aries and close to the Pleiades star cluster. At magnitude 5.8, it is visible in binoculars. Look for a tiny greenish disc compared to the pinpoints which are the stars. Neptune is seen against the background stars of Aquarius (below the asterism known as the Square of Pegasus) and lies due south on 4th.  At magnitude 7.8 it will require large binoculars or a small telescope to find it. Neptune displays a smaller and truly blue disc compared to that of Uranus although it will appear only as a bluish star to most small instruments

Meteor shower: There are two meteor showers due in December. The first, which peaks on the night of December 13, is the Geminids. The Geminid shower is the only shower associated with an asteroid rather than a comet. This asteroid is called Phaethon. The Geminid shower can be intense, but a bright Moon interferes with observations this year. The second shower this month is the Ursids which seem to emanate from a point in the constellation of Ursa Minor, which also contains the Pole Star. A nearly Full Moon also interferes with observations of this shower.

Comet: Comet Leonard is predicted to be the brightest comet of 2021 as it will be just two Astronomical Units from the Earth on December 12 (an Astronomical Unit is the distance from the Earth to the Sun, that is 93 million miles). The comet may then reach a peak of activity and brightness when it arrives at its closest point to the Sun (perihelion) on January 2, 2022. The comet can be found near the bright star Arcturus in the constellation of Boötes, on December 6. To find Arcturus, look north and find the Plough, then follow the handle of the Plough through its arc to Arcturus.

The New Moon occurs on December 4 with First Quarter on December 11, Full Moon then follows on December 20 and Last Quarter on December 27.

Data supplied by Simon Harding, Observations Secretary Torbay Astronomical Society

Diary dates

The next meetings of the Torbay Astronomical Society, complying with the current Covid regulations, will be held at Torquay Boys' Grammar School. On December 2, an observational evening will be held, weather permitting, when members will be delighted to provide views of the objects discussed in this article. In the event of bad weather, short talks and videos will be presented alongside informal discussion and an opportunity to get to know the society.

And on December 16, the society will hold its annual quiz and Christmas social evening. Talks will also be available online via Zoom.

Visitors and prospective members especially welcome. For details, contact the secretary on astrosecretary@gmail.com

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