Starry Skies Above Santa Monica

July 11-18, 2000

Mirek Plavec
Emeritus Professor of Astronomy, 
UCLA

   Great Event, But…
   What are the most exciting astronomical events? Let’s see: Eclipses of the Sun; eclipses of the Moon; bright comets; meteor showers…I think that occultations by the Moon of planets or bright stars come next. Among them, the occultation of Venus can be most impressive –- if it happens when the sky is sufficiently dark. It is this condition that is not fulfilled on Tuesday, July 17. Nevertheless, if the sky is sufficiently clear, the event can be observed with a small telescope, or even with binoculars. And from higher elevations (located above the densest and dirtiest part of our atmosphere), it might be possible to see it without an optical aid.
   The Moon, moving (with respect to the stellar background) from west to east, will cover Venus at 10:17 a.m. Venus will then re-emerge at 11:50 a.m. Where in the sky should you look for this event? The Moon will be a thin crescent, just three days before becoming the New Moon. The Sun will rise at 5:54 a.m. and culminate (reach the meridian, high above the south) at 1 p.m. The thin crescent Moon will precede the Sun: it will rise before 3 a.m. and culminate at 9:57 a.m. Since the occultation of Venus starts 20 minutes later, you should look for the crescent Moon high above the south. Venus will disappear behind the illuminated crescent of the Moon, and then re-emerge from behind its invisible dark limb. When Venus reappears, the Moon will already be above the southwest, but high above the horizon -– about as high as the Sun will be at that time.
   If the sky is clear, your chances of witnessing the event are good. If… you know our Santa Monica weather! 

   Planets Occulted, One By One, But…
   Actually, the occultation of Venus will be just one out of four similar events! Four planets are now projected into Taurus and close to the path of the Moon, and so they will be occulted one after another: Saturn before Venus on July 17, Jupiter on July 18, and Mercury on July 19! However, and if weather permits, we will be able to observe only the occultation of Venus. Saturn will be occulted for observers in South America, Jupiter for those in Indonesia, and the occultation of Mercury will occur for North Europe and Western Canada.
   Why are the regions of visibility so limited? The planets are sufficiently far away to appear projected into the same spot in the stellar background, no matter from where on the Earth you observe them. However, the Moon is much closer to the Earth: its distance (from the center of the Earth) around July 17 will be “only” about 373,000 km. As a consequence, when you, here in California, see the Moon projected in front of Venus, observers in South America will see the Moon pass above Venus, without any occultation. And for observers in Europe, both the Moon and Venus will be below the horizon when they meet. In the three other cases, we are the losers.
   This chain of occultations of four planets within a few days occurs very rarely. The Moon and each of the planets move in orbits whose planes are tilted to the plane of our orbit at various angles (fortunately rather small -– the greatest deviation is that of Mercury, 7 degrees.) The orbit of the Moon is inclined by somewhat more than 5 degrees. Since the disk of the Moon measures only about half-a-degree across, occultations of individual planets occur rather rarely, but occulting four planets during one lunation –- this is something really rare. This event is partly due to the (also rather unusual) fact that four planets happen to be projected into a fairly narrow area in the sky. 

   The Moon and the Planets On the Other Days 
   During the week of July 11 through 18, the illuminated portion of the lunar disk will be decreasing. After the Full Moon on July 5, the Last Quarter comes on July 13, and the Moon will be New on July 20. 
   Mars remains the only evening planet. When the sky gets sufficiently dark, after 9 p.m., you can see that Mars is already fairly high above the south-east. Just a few degrees to the west of Mars is its “competitor,” Antares (Anti-Ares), the brightest star in Scorpius. At present, Mars easily beats its competitor; although Mars already recedes from the Earth, it is still considerably brighter. 
   Mars culminates already shortly after 10 p.m. and sets in the southwest at about 3 a.m. Almost at the same moment, brilliant Venus emerges at the opposite side of the sky, and dominates the morning sky, which by that time is still fairly dark (clear signs of dawn appear around 4:30 a.m.) Venus can help you find Saturn. The faster moving Venus will pass fairly close to the south of Saturn on July 15. The relative motion of the planets with respect to the stars is rather slow, so you will see the two rather close to each other for several mornings around July 15. At dawn of July 17, the thin crescent Moon will join and adorn the show – a few hours before it actually occults Saturn and then Venus, as described above. 
   After 4 a.m., Jupiter joins the show, more submerged in the brightening sky, but sufficiently bright to be visible for at least half-an-hour. Jupiter has already passed from Taurus to Gemini, and just a bit farther to the east of it is Mercury, fainter than Jupiter but brighter than Saturn or any real star nearby. A very, very thin crescent Moon might perhaps be visible near Mercury on the morning of July 19, Thursday. 
   So there we are! So much is happening in the sky this week that the continuation of my story about discoveries of extrasolar planets must wait.