Scientific Space: Important Astronomical Phenomena in July 2011

Major Astronomical Events

01st 16:39 Partial Solar Eclipse
05th 03:15 Mars conjunct Aldebaran: 5.4^\circ N
07th 01:32 Mercury conjunct Beehive Cluster (M44): 0.2^\circ S
09th 05:44 Moon conjunct Spica: 2.6^\circ N
20th 12:59 Mercury Greatest Elongation: 26.8^\circ E
25th 23:28 Moon conjunct Pleiades: 2.2^\circ N
26th 20:46 Mercury conjunct Regulus: 2.8^\circ S
28th 00:52 Moon conjunct Mars: 0.5^\circ N
28th 21:47 Delta Aquariids Meteor Shower: ZHR = 20

Lunar Phases

01st 16:54 New Moon
08th 14:29 First Quarter
15th 14:39 Full Moon
23rd 13:02 Last Quarter
31st 02:40 New Moon

July 1 Partial Solar Eclipse

Among the four partial solar eclipses occurring globally in 2011, the one on July 1 has the poorest observation conditions. This is reflected in its very small visibility range and a maximum magnitude of less than 0.1. This partial solar eclipse is only visible over the ocean between Africa and Antarctica and a very small area of land in Antarctica. Since there is little human activity in this region, it is likely that very few people will observe it. If the magnitude of a partial solar eclipse is very small, the significance of observation is also limited. The maximum magnitude of this eclipse is only 0.097, and combined with the very low altitude of the Sun, the observation conditions are very unfavorable.

There are two necessary conditions for the formation of a solar eclipse. First is time, meaning a solar eclipse must occur on the day of the New Moon (the first day of the lunar month). Second is spatial position, meaning the Sun and the Moon must be near one of the lunar nodes (where the Moon’s orbit intersects the ecliptic). The former is related to the synodic month (29.530588 days), and the latter is related to the draconic month (the interval between two consecutive passages through the ascending or descending node, 27.212220 days). Therefore, the basic cycle of solar eclipses is the least common multiple of the synodic month and the draconic month, a cycle of approximately 18 years and 11 days, also known as the “Saros cycle.” If a solar or lunar eclipse occurs today, a similar eclipse will inevitably occur 18 years and 11 days later. Of course, the interval between adjacent solar eclipses is not that long because there are 42 independent solar eclipse systems occurring simultaneously, which we call Saros series.

If a solar eclipse occurs near the ascending node, the first eclipse in that Saros series begins in the high-latitude Arctic region of the Northern Hemisphere as a partial eclipse with a minimal magnitude. Every Saros cycle thereafter, the latitude moves southward and the longitude moves westward, with the magnitude gradually increasing. After 9 to 16 Saros cycles, a central eclipse (total or annular) forms. After another 42 to 48 Saros cycles, the eclipse path crosses the equator and moves south, entering the partial eclipse phase again with the magnitude gradually decreasing. After another 9 to 16 Saros cycles, the eclipse path approaches the South Pole, ending the series. Similarly, if a solar eclipse occurs near the descending node, the path moves in the opposite direction, from the South Pole toward the North Pole. A complete series lasts approximately 68 to 75 Saros cycles, or 1152 to 1440 years.

This partial solar eclipse marks the beginning of Saros Series 156, occurring near the descending node. This series contains 69 solar eclipses: the first 8 are partial eclipses in the Southern Hemisphere, the middle 52 are annular eclipses, and the final 9 are partial eclipses in the high latitudes of the Northern Hemisphere. It begins in 2011 and will end in 3237.

Obviously, this partial solar eclipse is far from China. It is worth reminding everyone that although China has seen several observable solar eclipses in recent years, this “eclipse observation season” is nearing its end. After the annular solar eclipse on May 21, 2012, it will be a long time before another annular or total solar eclipse is visible within China. Even a partial solar eclipse with slightly better observation conditions will not occur until 2016.

July 6 Mars Conjunct Aldebaran

Our neighbor Mars is a very distinctive planet in the solar system. Although its volume is only 1/7th of Earth’s, many of its properties are very similar to Earth’s. Mars does not emit light itself; we see it because it reflects sunlight. Based on material analysis of the Martian surface by probes, scientists have found that its soil and rocks are rich in fine particles of hematite or maghemite, which is likely the reason Mars appears red.

In the night sky, there are many celestial bodies that appear red. Famous bright red stars include Antares in Scorpius, Betelgeuse in Orion, and Aldebaran in Taurus. However, the reason these stars appear red is fundamentally different from Mars. When a star evolves to its final stages, the hydrogen fuel for nuclear fusion in its core is nearly exhausted. As the volume expands and the temperature drops, the star becomes a red giant. Subsequently, most stars with a mass similar to the Sun will shed some material to form a planetary nebula and eventually evolve into a white dwarf. A few stars with larger masses will undergo a supernova explosion, eventually becoming a neutron star or a black hole. Aldebaran in Taurus is a typical red giant, with a diameter about 38 times that of the Sun, and is currently entering its late stages.

In July, Mars is located in the constellation Taurus, rising from the east at approximately 2:00 AM with a magnitude of about 1.4. The observable time is gradually increasing. Aldebaran is often compared to the fiery eye of the Bull, with a magnitude of around 1.0. On July 6, Mars will be in conjunction with Aldebaran, sharing the same ecliptic longitude with an angular distance of about 5^\circ. In the low eastern sky before dawn, we can observe these two stars close together, both appearing somewhat red. It is worth mentioning that another bright red star near the ecliptic, Antares, also undergoes conjunctions with Mars. In ancient China, this was called “Yinghuo Shou Xin” (Mars lingering in the heart of Scorpius). We will have the opportunity to appreciate this phenomenon in October next year.

July 20 Mercury Greatest Eastern Elongation

The fourth greatest elongation of Mercury this year is the eastern elongation on July 20. Although the angular distance between Mercury and the Sun can reach 27^\circ during this elongation—the maximum for the year—Mercury’s declination in late July is 8^\circ to 10^\circ lower than the Sun’s, making observation conditions unsatisfactory. Taking the 40^\circ N latitude region as an example, at sunset on July 20, Mercury’s altitude is only 13^\circ with a magnitude of 0.4. If the atmospheric transparency in the low western sky is poor, it will be difficult to observe Mercury.

In contrast, 7 to 10 days before the greatest elongation, Mercury’s declination is closer to the Sun’s, resulting in a higher altitude at sunset and better observation conditions. If observing at 40^\circ N, Mercury’s altitude at sunset on July 11 can reach over 15^\circ. From this, we find that the best observation conditions for Mercury do not necessarily occur at the moment of greatest elongation; one must comprehensively consider the combined effects of its declination and angular distance from the Sun. In most cases, the highest altitude of Mercury at sunrise or sunset does not coincide with the moment of greatest elongation, and the same applies to Venus.

Sun, Moon, and Planets

Sun

At the beginning of the month, the Sun’s apparent right ascension and declination are 06h 38m 32.109s and +23^\circ 08’ 29.998”; at the end of the month, they are 08h 39m 32.135s and +18^\circ 23’ 59.067”. This month, the Sun moves from Gemini to Cancer. On the 1st at 17:00, a partial solar eclipse occurs, visible in the far southeastern Atlantic, far southwestern Indian Ocean, and a tiny part of Antarctica. On the 7th at 18:42, the solar term “Lesser Heat” (Xiao Shu) occurs when the Sun’s ecliptic longitude is 105^\circ. On the 23rd at 12:12, the solar term “Greater Heat” (Da Shu) occurs when the Sun’s ecliptic longitude is 120^\circ.

Moon

The times for perigee and apogee are July 7 at 22:00 and July 22 at 07:00, respectively. The phases of New Moon, First Quarter, Full Moon, Last Quarter, and New Moon are July 1 at 16:54, 8th at 14:29, 15th at 14:40, 23rd at 13:02, and 31st at 02:40. On the 28th at 01:00, there is a lunar occultation of Mars, with Mars located 0.5^\circ north of the Moon.

Mercury

Evening star, located low in the northwest at sunset. On July 20, Mercury reaches greatest eastern elongation with an altitude of 13^\circ at sunset and a magnitude of approximately +0.4. Although the angular distance from the Sun reaches 27^\circ, observation conditions are not ideal due to the low altitude. After the elongation, Mercury gradually approaches the Sun. On the 3rd at 10:00, Mercury is in conjunction with the Moon, 5^\circ north of the Moon.

Venus

Morning star. Close to the Sun and difficult to observe.

Mars

Mars is in prograde motion in Taurus. The time Mars rises in the east gradually advances to around 2:10 AM, with a magnitude of about +1.4. Observation conditions are gradually improving. On the 6th at 15:00, Mars is in conjunction with Aldebaran, 5^\circ north of Aldebaran.

Jupiter

Jupiter is in prograde motion in Aries, gradually moving away from the Sun. It rises in the east at approximately 0:00 AM with a magnitude of about -2.3. Observation conditions are further improving. On the 24th at 09:00, Jupiter is in conjunction with the Moon, 5^\circ south of the Moon.

Saturn

Saturn is in prograde motion in Virgo. In July, Saturn is located in the southwestern sky at sunset with a magnitude of about +0.9. It sets at approximately 11:00 PM, and the observable time is gradually decreasing. On the 8th at 12:00, Saturn is in conjunction with the Moon, 8^\circ north of the Moon.

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