Australian Sky & Telescope — July 2017

46 AUSTRALIANSKY&TELESCOPEJuly 2017

SUN, MOON & PLANETS by Jonathan Nally

Beacons of the dawn and dusk

Venus and Mercury shine at opposite ends of the day.

I

t’s now mid-winter and the

constellations of the galactic bulge

— Scorpius and Sagittarius and co.

— are high overhead, with their spread

of easy deep sky objects for binoculars

and small scopes. Down south, the

Southern Cross and the Two Pointers

are at maximum elevation just after

sunset — take a look at the Jewel Box

star cluster, always a favourite.

The combination of the ecliptic

(the plane in which the planets orbit)

standing up almost straight from the

horizon at this time of the year, together

with Mercury reaching its greatest

elongation (angular distance from

the Sun) on July 30, means that the

innermost planet will be ideally placed

for observation from mid-July to mid-

August, remaining above the western

horizon for about an hour after sunset at

the beginning of the month, and about

twohoursbymonth’send.OnJuly10th,

theplanetwillappearnexttotheBeehive

starclusterinCancer;andonthe25th,

lookforthethincrescentMoonnearby.

Mercury’s brightness will go from about

magnitude–0.5inearlyJulytoabout

+0.5 by the end of the month. During

that same period, its apparent diameter

will increase from about 5.5 arcseconds

to around 7.9 arcseconds.

Venuscontinuestoshinebrightlyto

theeastinthepre-dawnsky,wandering

amongthestarsofTaurus.There’ll

beaprettyspectacularsightonJuly

13, with the magnitude –4.0 planet

seemingtotaketheplaceofthestar

EpsilonTauri(alsoknownasAin)in

the constellation’s V-shaped head... and

fightingforattentionwithAldebaran

(magnitude0.8)justtotheright,and

thePleiadesstarclustertotheleft.Over

thefollowingweek,Venuswillcontinue

toslidetowardthehorizon;watchfor

theMoonaboveitonthe20thandthen

below on the 21st.

TLeft: VenuswandersamongthestarsofTaurusduringJuly.Right:You’ll need a clear horizon

tocatchtheMoon,MercuryandReguluslowonthehorizononJuly25.

The ‘Red Planet’, Mars, is still out

of view on the other side of the Sun

this month, and will remain so until

it reappears in our morning sky in the

middle of September.

Jupiter is still up and about,

visible high in the northern sky after

sunset, shining at magnitude –2.0 and

with an apparent diameter of about

36 arcseconds during July. The blue

star Spica, the brightest light in the

constellation Virgo, is above and to the

right of the planet. And watch for the

Moon very close below Jupiter on the 1st,

and close again on the 28th and 29th.

Saturn, which reached opposition in

June, is still very well placed for viewing

above the eastern horizon after sunset

and transiting (ie. being due north

and high in the sky) at 11:00pm at the

beginning of July, reducing to 9:00pm

by the end of the month. Shining at

an impressive magnitude +0.2 and

with an apparent diameter of about 18

arcseconds, it’s a great target for the

telescope... particularly this year with its

rings tilted nicely for our line of sight.

In fact, the angle of the rings and their

reflectivity are what is making Saturn so

bright at the moment, especially when

you consider that it is heading for its

aphelion (greatest orbital distance from

the Sun) next year. Look for the Moon

near the planet on July 7 and 8.

Although not visible to the naked

eye — in fact, you need a fairly large

telescope and accurate pointing — Pluto

will reach opposition on July 10. Can

you believe it is now two years since the

New Horizons spacecraft completed its

successful fly-by of the dwarf planet?

Finally, good old Earth will reach

its aphelion on July 4, at a distance

of152,092,565kilometres(equalto

1.016676 astronomical units) from our

nearest star.