An introduction to Jupiter and Venus through a recent conjunction
This past weekend’s close encounter between the planets Jupiter and Venus was a rare event. For those who were watching the western sky, just after dusk, the two planets came so close to each other as to appear to nearly touch, although still clearly visible as separate “stars” with the naked eye.
In astronomical terms, such a near conjunction is known as an “appulse”; and at their closest, the planets were within 4 arc minutes from each other.
An arc minute is a measure of distance in the sky, where 60 arc minute equals 1 degree. For the layperson, holding your fist at the full extension of your arm, while closing one eye, will block out about 10 degrees of sky. The earth’s rotation causes a relatively fixed object in the night sky (like a star) to move 360 degrees in 24 hours, or a quarter of a degree (or fifteen arc minutes) per minute.
On Saturday 27 August 2016, observers in the Western Hemisphere saw the planets at between 6 and 10 arc minutes apart; European and African viewers (like myself, from Johannesburg, South Africa) were treated to approximately 12-13 arc minutes; while Japan, East Asian and Australian viewers’ sight was still spectacular at 30 arc minutes or half a degree. If you had held a pencil at arms length, the back of the pencil would have blocked out the event. If you had been watching the stars against a fixed, clear horizon, it would have taken between 15 seconds and 2 minutes, depending upon where in the world you were watching, for the distance between the planets to have passed.
Proximity between Jupiter and Venus happens approximately every three to six years
(the difference is accounted for because not all proximities are visible), but not all encounters are the same: this last event will not repeat itself until 2065. This means that many of the readers of this article will likely not be alive to witness it again. Venus was also beautifully bright (because Venus is an inferior planet – closer to the sun than the earth – it waxes and wanes, like the moon) at above 90% illumination, making its conjunction with blue Jupiter all the more brilliant.
This appulse was still further unique by virtue of the fact that all five planets were visible during the same night :
to the left of the waltzing Jupiter and Venus, Mercury looked on wistfully; Mars and Saturn glowered ominously in a triangle with the constellation Scorpio’s red Antares from near zenith overhead.
Having all five planets visible during the same night only occurs for short periods of time - and we are in such a period at the moment. It requires that Jupiter and Saturn be within 180 degrees of each other, which excludes approximately 10 of the 20 years it takes for a grand conjunction between these august peers. Mars takes 686 days for its revolution, which roughly excludes alternate years of the possible 10. Venus is visible for two periods of its 224 day orbit: as the morning star for about 8 weeks, and as the evening star for a similar period about two months later. Finally, fleet-footed Mercury is only visible for two three-week periods (assuming there is nothing obstructing a viewing close to the horizon) every 88 days, which misses Venus’ appearance more than half of the time. Thus in a twenty year cycle there are only a handful of three week periods during an aggregate of five candidate years when all of the planets are visible during the same night.
The curiosity of this event calls into mind a similar but yet more spectacular appulse that occurred on 17 June 2BCE.
That conjunction has been proposed as being connected with the Star of Bethlehem – a controversial astronomic event recorded by Matthew in the New Testament. To some who believe in the account, the story is a remarkable illustration of how acting in faith can move persons to witness eternally significant events. To the skeptic, the event is no more than pious fable – perhaps even a fraud. Wherever one sits in relation to this debate, it has received attention from some of the greatest minds, including Johannes Kepler (1571-1630) and Sir Isaac Newton (1642-1727).
The appulse of 17 June 2BCE, viewed from the middle east at between 20:30 and 21:30, presented Jupiter and Venus within 36 arc seconds of each other. That is much closer than our conjunction, or within 0,01 degrees. It is a conjunction the degree of which has not been repeated in millennia.
The ready availability of planetarium software permits anyone with access to a computer to rewind the clock and view these events, as if there, on the night in question. In following posts, an evaluation of the truth claims of the gospel account will be undertaken. What will be presented may lay the basis for an argument that an extremely unique astronomic sequence of religiously laden events unfolded that could explain each of the essential features of the Star of Bethlehem story. It will provide a basis for evaluating between several competing claims that have been advanced in explanation of the phenomenon.
So, follow along; and if you can, download an available planetarium software (I use Stellarium a free download easily found through Google) and enjoy the ride! (the images are created using it)