Over to the west-northwest at dusk during June, a gathering is taking place between the two brightest ‘stars’ in the Northern Hemisphere sky, one that even casual stargazers cannot fail to notice. The ‘stars’ in question are, of course, planets — Venus (the brighter of the two) and Jupiter — and the stage is set for a spectacular denouement on the last day of June as the pair pass within a fraction of a degree.
Since its opposition in early February, Jupiter and its Galilean satellites have provided planetary observers with a wealth of interesting phenomena throughout the spring months. But as the Earth rounds the Sun, Jupiter is drawn inexorably into the summer evening twilight and we are fast approaching the end of this Jovian apparition. The largest planet passes from the constellation Cancer into Leo on June 10th.
Stealing the show at dusk is undoubtedly Venus, having passed from Gemini into Cancer on June 3rd. An unmissable naked-eye object even though it doesn’t attain greatest brilliancy (magnitude -4.5) until July 12th, the brightest planet is always a severe test for any telescope. But as it has rounded the Sun and drawn closer to Earth in recent weeks its small, gibbous disc has grown in angular size. Venus’ phase reaches dichotomy (or 50% illuminated, like a First Quarter Moon) on or about June 6th which is also the date that the planet reaches its greatest easterly elongation of 45.4° from the Sun.
Given the high precision of planetary position computations, you may be surprised by the phrase ‘on or about’ in the previous paragraph. While it is possible to calculate the instant that Venus appears half illuminated, observed dichotomy can differ from predictions by up to four days. This discrepancy was first noted by Johann Schröter (1748-1816) in August 1793 and the phenomenon is called the Schröter Effect. Widely accepted as merely an optical illusion, it is almost certainly due to Venus’ thick atmosphere. Does Venus appear half illuminated in your telescope on Saturday, June 6th?
Although a challenge to see in the dusk twilight of Northern Hemisphere skies, Venus passes close to the Beehive Cluster (Messier 44) in Cancer on the night of Saturday, June 13th. As seen from the centre of the British Isles, the sky is unlikely to be dark enough to see the planet just 0.6° (a little over the diameter of the Full Moon) away from the open star cluster until 11:30 pm BST, by which time the pair will be just 7° above the west-northwest horizon. Southern Hemisphere observers will have a better view of this particular conjunction.By June 15th, the angular separation of Venus and Jupiter has reduced to less than 10 degrees — this is about the span of a fist at arm’s length, or twice the field of view of a 10×50 binocular. By June 19th, observers with 7×50 or similar wide-field binoculars will be able to see both planets in the same field of view. Astrophotographers and general sky watchers may care to note that on Saturday, June 20th, the four-day-old crescent Moon will lie just 5.3° below Jupiter shortly after 11 pm BST.
As the angular separation of the two planets narrows in the last week of June, Venus crosses from Cancer to join Jupiter in Leo on June 26th. To observe the great finale of this conjunction on Tuesday, June 30th you need to find a location that offers an unobstructed and level view of the west-northwest horizon, because by the time the sky will be dark enough to see Venus and Jupiter just ⅓° apart with the naked eye they will be ~45 minutes from setting as seen from the centre of the British Isles.Observers with pre-aligned GoTo telescopes would be advised to setup shortly after sunset on June 30th and locate the planets while higher in the sky or, if you have an observatory-mounted instrument, to view Venus and Jupiter in the afternoon as they transit around 4 pm BST — taking every precaution not to accidentally view the Sun with tragic consequences for your eyesight. On July 1st, Venus and Jupiter will still be ½° apart. Have fun and clear skies!
Inside the magazine
You can find out more about observing the Solar System in the June edition of Astronomy Now in addition to a full observing guide to the night sky.
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