Starry Skies Above Santa Monica

June 19-26, 2002

Astronomy Picture of the Day

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Sun at its Highest…
   … above the celestial equator, and on our meridian at our local noon (12:55 p.m., summer time - PDT). The Sun will reach the point of “summer solstice” at 6:26 a.m. on June 21, Friday, and after that, it will slowly begin to move southward again, or so it appears to us. Actually, the Sun stands still, and it is our Earth, orbiting the Sun with its axis of rotation not perpendicular to the plane of its orbit, that causes the annual seasons for us. Thus, for us on the northern hemisphere of the Earth, summer begins on June 21.
   The day (when the Sun is above the horizon) reaches its longest duration, 14 hours and 25 minutes. Please check my calculations: the Sun rises at 5:42 and sets at 20:07, or, if you prefer, at 8:07 p.m. By the way, these times also mean 7 hours 18 minutes before 13:00 (1:00 p.m.), and 7 hours 7 minutes after 13:00. I hope that my using 13:00 instead of 12:00 does not surprise you – we have our summer time! The small remaining discrepancy between morning and afternoon is due to the fact that the Sun actually culminates at 12:55 PDT. Confusing? So forget about it – the life of an astronomer is full of these complications, but they mean very little to you in ordinary life

   The Full Moon at its Lowest
   This is perhaps a bit easier to understand: the Moon, when Full, should stand just opposite to the Sun. Therefore, when the Sun is highest above the celestial equator, between Taurus and Gemini, the Full Moon should lie lowest in the ecliptic. Provided, of course, that the Moon reaches its Full phase also exactly at the time of summer solstice, on June 21.
   This year, this condition is nearly satisfied, since the Full Moon comes on June 24. If it came on June 21, the Full Moon would be projected at the boundary between the constellations of Scorpius and Sagittarius. The Moon travels fairly fast, and the delay of about 3 days means that the Full Moon will be farther east along its orbit, and will shine above the main stars of Sagittarius –- to the east from the boundary with Scorpius.
    And there are more complications for the astronomer! The Moon does not move exactly in the same apparent orbit as the Sun. If it did, we would have a total eclipse of the Moon on June 24. Actually, the Moon will be located somewhat below the ecliptic. So it will miss the full shadow of the Earth, but will not escape completely: there will be a so-called “penumbral eclipse” of the Moon. For an observer on the Moon, the Earth will cover a small part of the solar disk, so that the Moon will be somewhat less illuminated by the Sun than it normally is. However, the radiation of the Sun is so powerful that this loss means little to an observer on the Earth: in principle, the Full Moon should be a bit fainter this time than it usually is. Naturally, the effects of our atmosphere (cloudiness, haze, fog) are much stronger, so the effect of the penumbral eclipse will be difficult to notice.

   More on Eclipses
   Somehow, I moved from the summer solstice to eclipses, so perhaps I can continue in that direction. I hope that at least some of you saw the partial eclipse of the Sun on June 10. If not, you probably saw a picture in the newspaper: the Sun’s disk reduced to a (still fairly thick) crescent similar to that of the Moon. Nothing too spectacular, but still worth seeing, I think.
   What is truly spectacular is a total eclipse of the Sun, when the Moon covers the Sun’s disk completely. The luminous disk of the Sun is covered, but you can see the solar corona surrounding it – a beautiful silvery aureole, and around the dark disk of the Moon, you may see the reddish prominence – geysers of hot gas erupting from the surface of the Sun.
   Please accept my strongest recommendation: If you can afford it, try to see the total solar eclipse – the most impressive phenomenon that has no bad consequences, as distinct, for example, from earthquakes…
   Why did I say “If you can afford it”? The reason is that the total solar eclipse is rather a rare phenomenon, and, unfortunately, for a number of coming years, it will require traveling pretty far from California. Here is why:
   The apparent disks of the two bodies have diameters so nearly identical that the total eclipse is extremely short, and never lasts longer than 7.5 minutes. And it is always visible from only a fairly narrow belt on the Earth – its width never exceeds 270 kilometers – but the belt can be quite long.
   The next total solar eclipse comes on December 4, and will be visible from South Africa and Australia. It will last 2.1 minutes. And the next good total eclipse after this one, on March 29, 2006, will require travel to North Africa or Asia Minor.

   The Royal Pair of Planets
   The two brightest planets, Jupiter and Venus, are still visible in the evening sky, but Jupiter is about to disappear. As I told you above, the Sun sets a few minutes after 8 p.m. Then the sky gets darker only slowly. The brightest stars become visible not earlier than after 9 p.m. Jupiter is brighter than any real star, but it will set already by 9:30. Look for it low above the north-western horizon. Venus is easier to see – it is brighter and higher above the western horizon; it sets after 10:30. These evenings are the most favorable for seeing this beautiful “Evening Star.”