Astronomy

May skies

Posted 2007 April 27 @ 10:00 - filed under Astronomy.

Some folk say that when it's truly dark, you can't see your hand in front of your eyes. Well, yes, if you're living in a cave, but under Karoo skies, there's a lot to be seen. For instance, two weeks ago, we (that's Carol, Gustav and myself) were in Sutherland observing the deepsky. At one point, Carol was reading notes by starlight; and later, I stood with my back to Jupiter and the Scorpius/Sagittarius Milky Way and wiggled my leg and we watched my shadow move on the ground. Now that's dark.

The thing is, one sees all these wonderful things not only because Sutherland is dark - but also because the eye is very sensitive. The eye's night-vision cells (rods) start to work when there is too little light to stimulate the cones, the primary daylight-vision cells. After about an hour's dark adaptation, you vision is 10,000 times more sensitive than during daylight. Ten thousand times! However, even slightly bright light disrupts this sensitive night vision: glimpsing a streetlight, a car's headlights, a mobile phone or a torch will affect your dark-sight.

On the outskirts of Sutherland town there are no bright lights, and if you set up behind a hill, then the town's few streetlights are hidden, and the occasional passing car remains unseen. The same caution can be taken if you're observing from city suburbs: set up your equipment where you are out of sight of all lights, except those from the stars above. In my suburban backyard I hang a series of dark curtains in a cordon around me, leaving me and my telescope isolated under the stars.

Even with dark-adapted vision, you still need some light to comfortably read star charts and jot down notes. A very, very dim red light is what you're after; lots of layers of red cellophane wrapped around a torch (kept in place with elastic bands) works well. Check out the image below, which I took two weeks ago in Sutherland.

Star gazers and trails over Sutherland. The bright star leading off the left edge is Procyon (Canis Minor), while the yellowish trail centre-top is Saturn. (40 minute exposure, 24mm f/5.6 lens, Kodak 400 ISO colour print film)

The camera's shutter was open for 40 minutes, and as the Earth turned the stars made beautiful curves. In the foreground my car is visible, revealed by the starlight. The two red blobs are from Gustav (centre) and Carol's (right) observing lights as they moved around during the long exposure.

The evening before this deepsky session we visited Jurg and Rita Wagener of "Sterland", where Carol showed the guests views through her telescope. We also used Jurg's two large 'scopes to enjoy the deep sights. Star clusters glistening like stellar jewels, massive clusters with a million suns, bright nebulae that would one day form stars and planets, and galaxies of 100 billion stars, passed before our eyes. And in our neighbourhood, brilliant Venus and ring-wrapped Saturn elicited a chorus of "oohs!" and "jiss!" from the guests.

Fortunately, you can still do a lot of star-gazing and deepsky observing even if the skies aren't perfectly dark. Last week I attended the ScopeX telescope and astronomy exhibition in Johannesburg, and could see half a dozen deepsky objects with my unadapted naked eye: from the middle of Randburg. All you need to get a really good view (from Sutherland or Johannesburg) is to give your eyes the change to dark adapt.

No dark adaptation is needed to see Venus, however, setting in the north-west this month at dusk. The Evening Star moves steadily towards Gemini, and is closest to Pollux on the 30th. Two nights later, Venus lines up with Castor and Pollux in the north-west. From the 14th onward, Jupiter and Venus can be seen at sunset, but on opposite horizons. Look for Jupiter low in the south-east at sunset. On May 24 (and again on June 02), Jupiter's four moons, readily seen in binoculars, are strung out on the same side of the planet, giving an interesting view. From about the 18th onward, Mercury can be seen low in the north-west; the small but bright planet climbs higher in the sky each evening.

May dusk evenings are characterized by Orion setting in the west, while the curved Scorpion rises in the east. On the evening of May 4, however, the Scorpion will rise without it's brightest star, Antares. Seen from southern Africa, the almost-Full Moon occults Antares when they rise. Shortly afterward, Antares will pop into view near the upper edge of the Moon (at 19:20 in Cape Town, 19:16 in Johannesburg, and 19:19 in Durban).

Speaking of the Moon, the 18th is the anniversary of the first astrophotograph taken of the Moon, in 1869 by Sir David Gill from Cape Town. With today's digital cameras, it's easy to take nice pictures of our Moon, as this tutorial explains. Why not try and photograph Antares re-appearing at the lunar limb? Other attractive photo opportunities are given in the diary below; full details in the May Southern Sky Almanack.

Sky diary for May

(full details in the May Southern Sky Almanack)

• Southern Sky Almanack (introduction)

• Southern Sky Almanack (May)

  
  

• Digicam photography of the Moon

• heavens-above.com for Iridium flares

  
  

Keywords: Astronomy, Southern Sky Almanack, 2007, monthly sky review

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