18 June 2008

Planetary Roundup

Three major planets are well placed for viewing. This chart of Jupiter and the Moon and Sagittarius shows the changing view of the trio as the Moon moves across the sky the evenings of June 18 and 19. Jupiter and Sagittarius will remain in close proximity over the summer with Jupiter being the brightest beacon in the sky each night. Over the summer Jupiter will gradually become visible earlier and earlier in the evening. Saturn and Mars are slowly slipping into the evening glare but remain visible for a short while after sunset. Within the next month they will entirely fade from view, so catch a glimpse and enjoy the show of these two wanderers in the midst of Leo the Lion.

04 June 2008

The Southern Sky

As I write this entry, I am in Melbourne, Australia on a business trip. Melbourne is approximately 38 degrees south of the equator and I have always likened this to San Francisco which is approximately 38 degrees north of the equator. I think of these two coastal cities having similar climate and similar sky conditions but in exactly the opposite way. Being 38 degrees north of the equator San Francisco is heading toward the summer solstice. The days are getting longer and the sky, as always, appears to pivot around the north star which is (by no coincidence) 38 degrees above the northern horizon, with the Sun and planets moving in a long arc across the southern half of the sky.

Melbourne presents a mirror image of the sky in many ways. The Sun does in fact still rise in the east and set in the west, but the Sun and planets move in a long arc across the *northern* part of the sky. The days here are short and getting shorter and Australia and other southern hemisphere countries prepare for the onset of winter in a few weeks. The mirror image of the sky in the northern hemisphere is visible in that the sky appears to pivot around the southern point of the sky. There is no bright star near the southern point of the sky so we don't have exactly the same scenario (ie. no south polar star like Polaris in the northern hemisphere), but the motions are similar -- in the opposite direction.

For example, in the northern hemisphere the Big Dipper moves around the north star in a large sweeping circle in a counter-clockwise direction, but in the southern hemisphere the southern constellations pivot around the south "pole" in a large *clockwise* circle every 24 hours. The Big Dipper and other star patterns and constellations near the north star are visible all night in the northern hemisphere and are called circumpolar stars, but are not at all visible from my location here in the southern hemisphere. Similarly, the circumpolar stars in the southern sky are not visible at all from San Francisco. Hence we never see the Southern Cross or Alpha Centauri (one of the nearest stars to Earth) or the Magellanic Clouds (nearby galaxies) from the northern hemisphere.

The early travelers setting sail across the seas and around the globe noted these changes in the sky and learned some valuable rules about celestial navigation. Although we don't depend upon these markers to tell us where we are on the globe anymore, I enjoy looking out for these changes in motion in the sky as I travel from time to time far north of San Francisco or in this case far south. The Southern Cross is a beautiful sight from here, even though the sky has been too overcast to see much else.