Monday, 21 December 2020

THE GREAT CONJUNCTION OF JUPITER AND SATURN

Joseph de Ca'th Lon
and The Grandma continue enjoying together talking about science.

Joseph likes Astronomy and they have been talking about the great conjunction of the planets Jupiter and Saturn that has occurred today. 

They have visited the Garraf Astronomical Observatory to enjoy this conjunction.

A great conjunction is a conjunction of the planets Jupiter and Saturn, when the two planets appear closest together in the sky.

Great conjunctions occur approximately every 20 years when Jupiter overtakes Saturn in its orbit. They are named great for being by far the rarest of the conjunctions between naked-eye planets.

The spacing between the planets varies from conjunction to conjunction with most events being 0.5 to 1.3 degrees. Very close conjunctions happen much less frequently, though the maximum of 1.3° is still close by inner planet standards: separations of less than 10 arcminutes have only happened four times since 1200, most recently in 2020.

On average great conjunction seasons occur once every 19.859 Julian years (365.25 days). This number can be calculated by the synodic period formula 1/(1/4332.59−1/10759.22) giving c. 7253.46 days -the average frequency of Jupiter overtaking Saturn from the Sun's POV due to the net effect of their 4332.59 and 10759.22-day orbits.

In practice Earth's orbit size can cause great conjunctions to reoccur up to some months away from the average time or the time they happen on the Sun. Since the equivalent periods of other naked eye planet pairs are all under 27 months this makes great conjunctions the rarest.

More information: BBC-Sky at Night Magazine

Occasionally there is more than one great conjunction in a season when they occur close enough to opposition: this is called a triple conjunction, which is not exclusive to great conjunctions.

The most recent great conjunction occurred on 21 December 2020, and the next will occur on 4 November 2040. During the 2020 great conjunction, the two planets were separated in the sky by 6 arcminutes at their closest point, which was the closest distance between the two planets since 1623.

The closeness is the result of one of the three approximately equally spaced longitude zones where great conjunctions occur shifting into the vicinity of one of the two longitudes where the two orbits appear to intersect when viewed from the Sun, which has a point of view similar to Earth.

The great conjunction zones revolve in the same direction as the planets at the rate of approximately one-sixth of a revolution per four centuries thus creating especially close conjunctions on an approximately four-century cycle. 
 
More precisely, the location in the sky of each conjunction in a series should increase in longitude by 16.3 degrees on average, making one full cycle relative to the stars on average once every 2,634 years. If instead, we use the convention of measuring longitude eastward from the First Point of Aries we have to keep in mind that the equinox circulates once every c. 25,772 years so longitudes measured that way increase slightly faster and those numbers become 17.95 degrees and 2,390 years.
 
The longitudes of close great conjunctions are currently about 307.4 and 127.4 degrees, in the constellations of Capricornus and Cancer respectively.

Earth's orbit can make the planets appear up to about 10 degrees ahead of or behind when they are at the optimal point, which also is true for any other part of their orbits.

Saturn's orbit plane is inclined 2.485 degrees relative to Earth's, and Jupiter's is 1.303 degrees.

Interestingly, the ascending nodes of both planets are similar, 100.6 degrees for Jupiter and 113.7 degrees for Saturn, so that if Saturn is above or below Earth's orbital plane Jupiter usually is too, this is partly caused by Earth's orbit being tilted relative to all the large planets.

Because the orbit inclination directions of Jupiter and Saturn align reasonably well it would be expected that no closest approach will ever be much worse than Saturn's orbit tilt (2.485°) minus Jupiter's (1.303°). Indeed, between the year 1 and 3000, the maximum conjunction distances were 1.3 degrees in 1306 and 1940.

More information: Time and Date

Conjunctions in both years occurred when the planets were tilted most out of the plane: longitude 206 degrees (therefore above the plane) in 1306, and longitude 39 degrees (therefore below the plane) in 1940.

When studying the great conjunction of 1603, Johannes Kepler thought that the Star of Bethlehem might have been the occurrence of a great conjunction. He calculated that a triple conjunction of Jupiter and Saturn occurred in 7 BC, −6 using astronomical year numbering.

A triple conjunction is a conjunction of Jupiter and Saturn at or near their opposition to the Sun.

In this scenario, Jupiter and Saturn will occupy the same right ascension on three occasions or same ecliptic longitude on three occasions depending on which definition of conjunction one uses, this is due to apparent retrograde motion and happens within months.

The most recent triple conjunction occurred in 1980 and 1981 while the next will be in 2238 and 2239. Lights in the sky that look like especially bright stars are commonplace. Jupiter & Saturn only appear to be in the same spot for a moment, but they spend weeks approaching and departing from each other.

They are visibly separate, distinct objects for the vast majority of the same evening on which the conjunction occurs, something wise men are not likely to miss. Aside from the 7 BC conjunction occurring at the wrong time, the Star of Bethlehem as described moves west, but makes a sharp turn to the south, a movement no star or planet can actually make.

Great Conjunctions never appear to be leading southward to any particular building in Isreal. Additionally, there are other, better candidates for the Star ofBethlehem.

The astronomers from the Cracow Academy, Jan Muscenius, Stanisław Jakobejusz, Nicolaus Schadeck, Petrus Probosczowicze, and others observed the great conjunction of 1563 to compare Alfonsine Tables (based on a geocentric model) with the Prutenic Tables (based on Copernican heliocentrism).

In the Prutenic Tables the astronomers found Jupiter and Saturn so close to each other that Jupiter covered Saturn, actual angular separation was 6.8 minutes on 25 August 1563.

The Alfonsine Tables suggested that the conjunction should be observed on another day but on the day indicated by the Alfonsine tables the angular separation was a full 141 minutes.

The Cracow professors suggested following the more accurate Copernican predictions and between 1578 and 1580 Copernican heliocentrism was lectured on three times by Valentin Fontani.

More information: Space

The great conjunction of 2020 was the closest since 1623 and eighth closest of the first three millennia AD, with a minimum separation between the two planets of 6.1 arcminutes. This great conjunction was also the most easily visible close conjunction since 1226, as the previous close conjunctions in 1563 and 1623 were closer to the Sun and therefore more difficult to see. It occurred seven weeks after the heliocentric conjunction, when Jupiter and Saturn shared the same heliocentric longitude.

The closest separation occurred on 21 December at 18:22 UTC, when Jupiter was 0.1° south of Saturn and 30° east of the Sun. This meant both planets appeared together in the field of view of most small and medium sized telescopes, though they were distinguishable from each other without optical aid.

During the closest approach, both planets appeared to be a binary object to the naked eye. From mid-northern latitudes, the planets were visible one hour after sunset at less than 15° in altitude above the southwestern horizon in the constellation of Capricornus.

The conjunction attracted considerable media attention, with news sources calling it the Christmas Star due to its proximity to Christmas.

More information: Earth Sky


 We must believe then, that as from hence
we see Saturn and Jupiter;
if we were in either of the Two,
we should discover a great many Worlds
which we perceive not;
and that the Universe extends so in infinitum.

Cyrano de Bergerac

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