Jupiter dom­i­nates evening sky

The Guardian (Charlottetown) - - COMMUNITY - Glenn Roberts Glenn K. Roberts lives in Strat­ford, P.E.I., and has been an avid am­a­teur astronomer since he was a small child. His col­umn ap­pears in The Guardian on the first Wed­nes­day of each month. He wel­comes com­ments from read­ers, and any­one who would

Mer­cury puts in a brief ap­pear­ance at dusk this month, low above the WNW hori­zon about 30 min­utes af­ter sun­set.

It be­gins July shin­ing at mag. -1.0, but fades, by the end of the month, to mag. +0.4. On the evenings of July 24 and 25, about 30 min­utes af­ter sun­set, look for the thin, wax­ing, cres­cent moon near Mer­cury and the +1.0 mag. star Reg­u­lus, in the con­stel­la­tion of Leo the Lion.

As the early evening sky dark­ens, mighty Jupiter comes into its own. The bright­est evening planet of the sum­mer, Jupiter shines at mag. -2.0 as July be­gins, fad­ing only slightly to mag. -1.9 by month’s end.

Jupiter sets around mid­night dur­ing the first half of July, but dis­ap­pears be­low the west­ern hori­zon about two hours ear­lier by month’s end. Look for Jupiter’s four largest moons Io, Europa, Ganymede and Cal­listo - as they whirl around the planet from hour to hour and night to night. On July 28, the cres­cent moon sits above Jupiter.

Af­ter its op­po­si­tion (and clos­est ap­proach to Earth for the year) on June 15, Saturn ac­tu­ally tran­sits (reaches its high­est point in the sky for the night) higher in the night sky this month than it did in June, reach­ing be­tween 25 and 30 de­grees above the south­ern hori­zon shortly be­fore mid­night as July be­gins, and around 9 p.m. at month’s end. Saturn’s mag­nif­i­cent ring sys­tem is open to 26.7 de­grees (their near-great­est pos­si­ble tilt) as seen from Earth. On the night of July 6, the wan­ing, gib­bous moon sits above Saturn in the SSE sky. Though you will need a tele­scope to see the rings, binoc­u­lars will en­hance your view of Saturn’s golden colour.

Venus, our morn­ing star, rises about two and a half hours be­fore the sun as July opens (three hours by month’s end), and dom­i­nates the dawn sky. This bril­liant planet fades only slightly this month, from mag. -4.2 to mag. -4.0. About one hour be­fore sun­rise on the morn­ing of July 14, Venus will sit to the up­per left of the bright star Alde­baran (in Taurus - the Bull) in the east­ern sky. Look for the wan­ing, cres­cent moon to the lower right of Venus on the morn­ing of July 20, about 45 min­utes be­fore sun­rise.

Mars is near­ing su­pe­rior con­junc­tion (pass­ing on the far side of the sun as seen from Earth) on July 27 and is cur­rently lost in the set­ting sun’s glare. It will reap­pear in the east­ern dawn sky in late Septem­ber or early Oc­to­ber. In ex­actly one year, July 27, 2018, Mars will pass on the op­po­site side of the Earth as seen from the sun, at which time, it will be very close to Earth, and be­come the fourth­bright­est ce­les­tial ob­ject in the night sky af­ter the sun, moon and Venus.

A long-lived shower (July 12 - Aug.23), the Delta Aquar­ids (ra­di­ant in Aquarius - the Wa­ter Bearer) have a nom­i­nal peak date of July 27-28.

This shower is thought to be de­bris from Comet 96P Mach­holz, though not known for cer­tainty. The meteors strike the Earth’s up­per at­mos­phere about 100 km high and, at ap­prox­i­mately 150,000 km per hour, of­ten leav­ing per­sis­tent trails of glow­ing ion­ized gas for sev­eral sec­onds af­ter the me­teor winks out. The wax­ing, cres­cent moon sets be­fore mid­night on July 27, leav­ing the pre-dawn sky moon-free for the peak some­where be­tween 3-4 p.m. on July 28. Ex­pect to see about 20 plus meteors per hour ra­di­at­ing out­ward from the south­ern sky un­der a dark sky away from city lights. The Aquar­ids may over­lap with some early, brighter Per­seid meteors. They peak in mid Au­gust.

Here’s an in­ter­est­ing ce­les­tial fact: On July 3, the Earth reached aphe­lion, the far­thest point from the sun in its an­nual or­bit. Yes, we are ac­tu­ally about 3.3 per cent fur­ther from the sun, at this time of the year, than we are in Jan­uary, when we reach per­i­he­lion (clos­est point to the Sun).

Un­til next month, clear skies.

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