Ein Beispiel für dieBenutzung von

virtuellen Observatorien

Planetary Conjunctions, theStar of Bethlehem and theEnd of the World in 2012

information and contacts: http://vo-for-education.oats.inaf.it - iafrate@oats.inaf.it

Florian Freistetter, ZAH, Heidelbergflorian@ari.uni-heidelberg.de

Our project depends on your support. If you foundour material useful, we kindly ask you to

acknowledge it in your publications, or to write us an email (iafrate@oats.inaf.it), orlike it on our Facebook page (www.facebook.com/VOedu). Thanks!

Within this use case you will learn about an event that has been recorded by astronomers forthousands of years. Planetary conjunctions are events that are easy to find mentioned even outsidescientific literature. You explore the motion of planets both around the Sun and in the sky, learnabout planetary conjunctions and try a guess at what really might have been the Star of Bethlehem.

If used in the classroom this basic use case is technically very simple. Before performing it,however, the teacher has to provide a simple geometrical model of the solar system.

1

The motion of the planets

The planets of our Solar System have beenformed in a large disk of gas and dust thatsurrounded the young Sun.

Because all the planets come from thesame disk, their orbits are all more or lessin the same plane – that of the disk!

When viewed from the Earth, it looks like asif all planets follow a line on the sky whenthey move in front of the background stars.

This line is called „ecliptic“ and it is definedby the apparant path of the Sun along thecelestial sphere – which is really nothingelse than the path of the Earth around thesun.

Because we observe the motion of theplanets from a moving planet – the Earth –the apparent motion of the other planets onthe sky can be quite complex.

Especially notable are conjunctions: anevent when two celestial bodies (in ourcase planets) are very close to each otherin the sky.

Of course this does not mean, that theplanets are also close to each other inreality – it only appears to us in such a wayfrom the Earth.

Conjunctions have always been of greatinterest to people; especially to astrologers.In former times they thought that theplanets are symbols of the gods or the godsthemselves and when they meet on the sky,the will fight each other or discus the fate ofthe humans and thus conjunctions couldherald good or bad news.

Astrology is, of course, nonsense – butplanetary conjunctions are still populartoday. Many people still believe in oldsuperstitions and think of certainconjunctions as a harbinger of doom.

Observing the motion ofplanets with Stellarium

Stellarium (http://www.stellarium.org) is afree programm to simulate the sky and themotion of the celestial objects.

One can specify a certain location and a

certain date and then watch how the skywould like for this special specifications:

The objects in the sky move in real time –but of course one can accelerate the motionusing the controls of stellarium.

It is also possible to search for any object inthe sky:

For a quick demonstration, lets use theplace of your current location and thecurrent time and search for the planetJupiter. If Jupiter appears on the sky,depends on its current visibility. Maybe it isonly „visible“ during the day? Then the lightof the sun would be to strong for a planet tostill be observable.

2

Figure 1: Specify Time and Location

Figure 2: Control the speed of time

Figure 3: Search an object

In Stellarium, you could „switch off“ theatmosphere:

Then you can see the sky as it wouldappear during the day if no sunlight woulddisturb our view. You can now center onJupiter - (use the symbol) -and then accelerate the time.

Jupiter moves around the sky on a certainpath which is very close to the ecliptic. Youcan let Stellarium display the ecliptic bypressing the „,“-key.

Lets follow the motion of the other planets(Mercury, Venus, Mars, Saturn, Uranus,Neptung). Do all of them follow the eclipticexactly? Who shows the greatesdeviations?

The Star of Bethlehem

Some scientists believe that the „Star ofBethlehem“ as described in the bible was areal celestial event. Some astronomershave proposed that it was a conjunctionbetween some planets that was laterdescribed as a „star“

One possibility that could be the basis ofthe „Star of Bethlehem“-myth is aconjunction between Jupiter and Saturnthat happend on 12. September of the year7 BC.

Use Stellarium and try to watch the sky as ithas looked like in ancient Israel. But notethe following points:

● If you want to use the city ofJerusalem as a location: inStellarium it is stored under the

hebrew name „Yerushaláyim" (ofcourse you can use also any othercity in the area).

● There was no year zero! December31st of the year 1 BC is directlyfollowed by January 1st of the yearone. However, Stellarium has a yearzero. The year 7 BC is thus the year"-6" in Stellarium.

●

An other possible conjunction that the „starof Bethlehem“ could be based on happendin the year 3 BC on August 12 th. ThereVenus and Jupiter appeared to be almost inthe same place of the sky.

A planetary alignement in2012?

People and especially the media often talkabout „planetary alignements“. They mean,that the planets would all aligne along a lineon the sky which would result in variousdesasters (floods, earthquakes, etc).

Depending on how the planets aligne, itwould look different from earth. If theplanets appear along a line on the sky, thenthere real positions in the solar system ofcourse are not aligned. If thatwas the case,i.e. if all planets form a „real“ line (e.g. if you

3

Figure 4: Controlling the View

Figure 5: Saturn and Jupiter in the year 7 BC

Figure 6: Venus and Jupiter one day before conjunction

could look on the solar system from above),then from Earth it would look like as if allplanets would be on the same point in thesky.

But such a situation will never occur. Thechances for such an alignement are sosmall that even the lifetime of the wholeuniverse is way to short for it to happen.

Some people claim that such a conjunctionwill happen on December 21st of the year2012 and that the combined gravitationalforce of the aligned planets will cause greatcatastrophies.

With Stellarium you can easily check thatclaim. You will see, that there is no suchalignement on December 21st, 2012.Mercury and Venus come a little bit close toeach other but nothing more happens!

You can also try to calculate the forces ofthe planets acting on Earth. Jupiter is thelargest planet in the solar system; 300times more massive than Earth. UseNewtons law and calculate its force onEarth. Then calculate the force the Earthfeels from the Moon. You will see that theforce from the Moon is much larger –Jupiters force is a hundreds time weaker(because Jupiter is far away from Earth).The forces of the other planets are evenweaker. There position in the sky has norelevant influence on the force they areacting on the Earth! Even if a „line ofplanets“ would happen – it could not causea catastrophe on Earth!

We habe already seen, that all planetsmove along a line on the sky – the ecliptic.So even if the planets do not appear as aline in the real solar system, it sometimescan happen that they appear to be close toeach other on the ecliptic.

Such an event happend in May 2000. Therethe planets Mercury, Venus, Mars, Jupiterand Saturn and the Sun and the Moon werevery close to each other.

You can use Stellarium to look at thisphenomenon. Since all the objects werevery close to the Sun, it was not possible toobserve the conjunction on the real nightsky in 2000. When the sun had set, all theplanets would have set with the sun andduring the day the Sun would be to bright toobserve the planets. And in Stellarium youhave to „switch off“ the atmopshere too toobserve the planets close to the sun.

4

Figure 7: The sky on Dec. 21st, 2012

Figure 8: Planetary Alignement in 2000