With the arrival of autumn, the Great Square of Pegasus has gradually replaced the Summer Milky Way as the protagonist of the night sky. Although the autumn night sky has few bright stars and feels somewhat lonely, deep-sky objects like the Andromeda Galaxy are undoubtedly favorite targets for amateur astronomers. Additionally, observation conditions for Jupiter are excellent this month. Sporadic meteors and meteor showers have also entered a period of high activity, with the September epsilon-Perseids being particularly noteworthy.
Celestial Dance
02nd 02:00 Venus in conjunction with Spica
03rd 21:00 Mercury at inferior conjunction
04th 22:00 Mars in conjunction with Spica
10th 06:00 Saturn in conjunction with the Moon
11th 16:00 Mars in conjunction with the Moon
11th 21:00 Lunar occultation of Venus
12th 11:00 Mercury stationary
20th 01:00 Mercury at greatest western elongation
21st 00:00 Neptune in conjunction with the Moon; 18:00 Jupiter in conjunction with the Moon
21st 20:00 Jupiter at opposition
22nd 01:00 Uranus at opposition
23rd 03:00 Jupiter in conjunction with Uranus; 19:00 Jupiter in conjunction with the Moon, Uranus in conjunction with the Moon
24th 18:00 Venus at greatest brilliancy
29th 14:00 Venus in conjunction with Mars
On the Moon
02nd 01:21:46 Last Quarter
08th 18:29:48 New Moon
15th 13:49:44 First Quarter
23rd 17:17:12 Full Moon
Solar Terms
08th 01:44:40 White Dew (Bailu)
23rd 11:09:02 Autumnal Equinox (Qiufen)
Day and Night Conditions on the 15th
Sunrise 06:17:33 Sunset 18:34:56 Transit 12:26:25
Moonrise 13:04:02 Moonset 23:58:19 Lunar Transit 18:30:56
Civil Twilight Start 05:55:04 Civil Twilight End 18:57:24
Daylight Duration 12:17:23 Daytime Duration 13:02:20
September 9: Maximum of the September epsilon-Perseids
As we enter the second half of the year, the activity of meteor showers with larger flow rates increases significantly. While the reasons for this are not yet definitive, these months are definitely a great opportunity to appreciate and photograph beautiful meteors. Several meteor groups active in September have low flow rates, mainly the \alpha-Aurigids (AUR), the \delta-Aurigids (DAU), and the September epsilon-Perseids (SPE). Among them, the \alpha-Aurigids arrive first, with an active period from late August to September 3. This is followed by the September epsilon-Perseids, active from September 5 to 13. Closely following is the \delta-Aurigids, which can be seen until mid-October.
By analyzing historical observation data of these three meteor groups, scientists have found similarities in their radiant positions, meteoroid velocities, and colors. Therefore, the International Meteor Organization (IMO) now collectively refers to them as the Near-Aurigid Showers. In most cases, even during the maximum, the hourly rate of these groups is only in the single digits, with only the \alpha-Aurigids having experienced an outburst in 2008. This year, the maximum of the September epsilon-Perseids is expected to occur on September 9. This period coincides with a small moon age. If astronomical enthusiasts plan activities such as astrophotography, they might consider observing the Near-Aurigid showers as one of their projects.
From the perspective of the radiant point, the September epsilon-Perseids can be observed almost all night, but the radiant point does not reach a high altitude until after 23:00 local time. According to the IMO forecast, the maximum will occur around 23:00 Beijing time on the 9th. There will be no moonlight interference throughout the night, making for good observation conditions. However, due to the low flow rate and the high number of sporadic meteors in the latter half of the night, it is difficult to determine if a meteor belongs to this specific group even if one is observed.
September 20: Mercury at Greatest Western Elongation
After the greatest eastern elongation on August 7, Mercury’s altitude at sunset decreased rapidly as it approached the Sun. By the inferior conjunction on September 3, Mercury was completely submerged in the Sun’s glare. Afterward, Mercury became a morning star again, and its altitude at sunrise rose quickly. By dawn in mid-September, it will appear in the southeastern sky. On September 20, Mercury will reach its third greatest western elongation of the year, with an angular separation of 18^\circ from the Sun. On that day, Mercury’s altitude at sunrise will be approximately 16^\circ, with a brightness of -0.4 magnitude. This is the second best opportunity to observe Mercury this year, following the greatest eastern elongation on April 9. Subsequently, Mercury’s altitude will decrease slowly, and throughout late September, as long as the weather is clear, we may be able to find it before dawn.
September 21: Jupiter at Opposition
In terms of both mass and volume, Jupiter is the leader among the planets in the solar system. However, when observed from Earth, it is not as bright as Venus, lacks the beauty of Saturn’s rings, and lacks the distinct personality of Mars. In ordinary telescopes, it is difficult to observe details like the Great Red Spot on Jupiter’s surface; the main attractions are the colored cloud belts and the four bright Galilean moons.
Jupiter’s orbital radius around the Sun is about 780 million kilometers. Its distance from Earth ranges from about 930 million kilometers at its furthest to about 620 million kilometers at its closest. Because the distance is so vast, the change in its apparent size is not significant at different times. Therefore, observation conditions for Jupiter are good for several months around the opposition. Often, the most important factor affecting observation is its altitude when it transits the meridian.
We know that the ecliptic and the celestial equator are not parallel, so the altitude of the ecliptic at transit varies for different locations. For mid-latitude regions in the Northern Hemisphere, the altitude is lower when the ecliptic passes through Scorpius and Sagittarius, and higher near Taurus and Gemini. In regions between the Tropics, the ecliptic can even pass through the zenith. Areas with lower altitudes, commonly referred to as the low sky, are more affected by the atmosphere, resulting in poorer observation conditions. This is why a 0-magnitude star appears dim near the horizon but very bright near the zenith. In previous years, Jupiter was located in Scorpius and Sagittarius, so its altitude was low even during midnight transits at opposition. Recently, Jupiter has moved from Aquarius to Pisces. At this opposition, its angular distance from the vernal equinox is only a bit more than 1^\circ, with a brightness of -2.9 magnitude, making observation conditions increasingly better.
During the Jupiter opposition on September 21 this year, another planet nearby—Uranus—is also a major highlight. Uranus is very close to Jupiter in angular distance at this time, making it convenient for us to find it with a telescope.
September 22: Uranus at Opposition
Just one day after Jupiter’s opposition, Uranus also reaches its opposition position. To be precise, it is the relative position of Uranus and Earth that forms the opposition. At 01:00 on September 22, Uranus will have a brightness of about 5.7 magnitude. Its position on the celestial sphere is almost exactly opposite to the Sun. It rises in the east when the Sun sets on the 21st and sets in the west at sunrise the next day, making it observable all night.
The average distance from Uranus to the Sun is 2.869 billion kilometers. In most cases, its brightness is around 6th magnitude, making it difficult to distinguish from other stars with the naked eye. Therefore, before the invention of the telescope, people were unaware of this planet’s existence. On March 13, 1781, British astronomer William Herschel discovered it using a self-made telescope. Initially, Herschel thought it might be a comet, but later scientists confirmed it was a planet. The discovery of Uranus doubled the known scale of the solar system. However, most of our current knowledge comes from the 1986 Voyager 2 flyby. From Earth, Uranus has an apparent diameter of only 4'', less than 1/4 of Saturn’s, making it difficult to capture its disk with small telescopes.
Jupiter orbits the Sun in about 12 years, while Uranus takes 84 years. They come close in the sky every 14 years. Due to retrograde motion, they can have three conjunctions in one cycle. This period, Uranus and Jupiter are "very close." The first conjunction was on June 7, the second is on September 23, and the third will be in early January next year. The September 23 conjunction has the largest angular distance of the three, about 50'. Finding Jupiter in the relatively dim Pisces is easy, and from there you can use a star chart to find Uranus.
September 24: Venus at Greatest Brilliancy
For regions at 40^\circN latitude, during this appearance of Venus to the east of the Sun, its highest altitude at sunset was in late May, greatest eastern elongation was on August 20, and its maximum brightness is reached on September 24. Venus does not emit light; it reflects sunlight. It is bright because of its high atmospheric albedo. Its brightness depends on its distance from Earth and its phase. Greatest brilliancy occurs between greatest elongation and inferior conjunction. The brightness at greatest elongation is about -4.5 magnitude, while at its brightest it can reach -4.9 magnitude. If the atmosphere is clear, Venus can be strikingly bright at dawn or dusk.
September offers many highlights for Venus. Throughout the month, it remains close to Mars and Saturn. On September 11, Venus is in conjunction with the crescent Moon, and on September 29, it will be in conjunction with Mars.
Dynamics of the Sun, Moon, and Planets
- Sun
-
At the beginning of the month, the Sun’s right ascension and declination are 10h 40.4m, +8^\circ 23.7'; at the end of the month, they are 12h 24.7m, -2^\circ 40.3'. The Sun moves from Leo to Virgo this month.
- Moon
-
The Moon passes perigee on September 8 at 12:00 and apogee on September 21 at 16:00. Phases: Last Quarter on the 2nd at 01:22, New Moon on the 8th at 18:30, First Quarter on the 15th at 13:50, and Full Moon on the 23rd at 17:17.
- Mercury
-
On September 3, Mercury is at inferior conjunction and hidden in the Sun’s glare. Before the 3rd, it is an evening star; after the 3rd, it becomes a morning star, rapidly moving away from the Sun. It appears in the northeast before dawn and moves to the southeast by mid-September. On September 20, it reaches greatest western elongation (18^\circ), with an altitude of 16^\circ at sunrise and a brightness of -0.4 magnitude. This is the second best time to observe Mercury this year.
- Venus
-
Evening star, moving through Virgo. It is low in the southwest at sunset, with a brightness of about -4.4 magnitude. On September 24, it reaches its maximum brightness of -4.8 magnitude. On the 2nd at 02:00, it is in conjunction with Spica (1^\circ south). On the 11th at 21:00, there is a lunar occultation of Venus (0.3^\circ north of the Moon). On the 29th at 14:00, it is in conjunction with Mars (6^\circ south).
- Mars
-
Moving from Virgo to Libra. At sunset, it is in the southwest, with its altitude decreasing from 17^\circ to 13^\circ and a brightness of 1.5 magnitude. It sets around 19:30, making it difficult to observe. On the 4th at 22:00, it is in conjunction with Spica (2^\circ north). On the 11th at 16:00, it is in conjunction with the Moon (5^\circ north).
- Jupiter
-
Retrograding in Aquarius. In September, it rises in the southeast after sunset and is visible all night with a brightness of -2.9 magnitude. This is the best time of the year to observe Jupiter. On the 21st at 18:00, it is in conjunction with the Moon (5^\circ south). Opposition is on the 21st at 20:00. On the 23rd at 03:00, it is in conjunction with Uranus (0.9^\circ south). On the 23rd at 19:00, it is in conjunction with the Moon (7^\circ south).
- Saturn
-
Moving through Virgo. At sunset, it is in the southwest and sets shortly after, making it difficult to observe. On the 10th at 06:00, it is in conjunction with the Moon (8^\circ north).
(Data from Astronomical View 2010, Amateur Astronomer, Stellarium)
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