September Skies

Capricornus and Cepheus are well-placed for observation this month.


Sun: The equinox occurs on the 23rd, at 02.29 GMT. This is 20.29 MDT on the 22nd in Denver. The Sun now moves below the equator on its yearly journey.

Moon: First Quarter, 2nd; Full, 9th;Last Quarter, 16th; New, 24th.. This is the Harvest Moon, the full moon closest to the equinox. Apogee the 20th, perigee the 8th.

Mercury: visible after sunset all month. .

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Venus: approaching conjunction with the Sun in the dawn..

Mars: in the evening sky an hour east of Saturn.

Jupiter: in the morning sky about 2-1/2 hours ahead of the Sun.

Saturn: in the evening sky drawing away from Mars.

September offers the best condtions of the year for observing the night sky. The evenings are still warm, and the Sun sets at about 6.30 pm and rises about 5.30 am, giving 11 hours of night. The weather is usually clear and dry. Only autumnal hazes interfere (aside from the constant light pollution of the city). If you have a country location, this is a good time to lie on your back in the darkness and watch the sky all night. As the sun sets, Virgo to Pisces will be displayed; at midnight, Aquila to Taurus, and towards dawn, Pegasus to Leo. Practically the whole sky will pass in review through the night. The descriptions here are for 9 pm, standard time (10 pm daylight time) around the middle of the month. Every two hours that elapses shows the stars as they will be a month later. I observed these September stars in July.

In the evening, however, the Summer Triangle is just past the meridian, which is now about 21h right ascension (the local Sidereal Time). South of the equator, Capricornus, the Sea Goat, is before our eyes. The ecliptic slants upward from Sagittarius to pass directly over Capricorn, so it is not unusual to find planets in the area. At present, Uranus is about 2/3 of the way from δ Aqr to γ Psc (both rather dim), while Neptune is near δ Cap. Uranus can be seen in binoculars, but it is only 5th magnitude, so is easily confused with stars. To be sure you have seen it, you must be familiar with all the stars in the area to notice the interloper. These planets have not moved far since last September, but are noticeably eastward of where they were then.

Another triangle can provide landmarks in the September skies, The September Triangle of Altair, Enif (ε Pegasi) and Sa'ad al Su'ud (β Aquarii), sketched above. There is no problem finding Altair and its guards. Enif is directly east along the 10° N parallel of declination, the next bright star in that direction. Its name is "fine" spelled backwards, but more probably comes from Al Anif, "the nose." It represents the nose of Pegasus, of course. Sa'ad al Su'ud completes a nice isosceles triangle. It's the least bright of the three, but is still easy to find, as the westernmost of the two principal stars of Aquarius. The name means "Luckiest of the Lucky." The line between Altair and Enif passes by Delphinus and Equuleus, equally spaced at 1/3 and 2/3 of the way. Delphinus is just north of the line, while Equuleus is just south of it.

Equuleus is a small, dim constellation well away from the galactic equator, so it does not contain wonders as the equally small constellation Scutum, located in the Milky Way, does. It is not too difficult to find by sweeping with binoculars west from Altair through Delphinus, looking for the pairs of stars making up the constellation at about halfway between Delphinus and Enif. The quadrilateral α-β -δ-γ can be made out, with ε to the southwest. α is named Kitalpha. Equuleus means "foal" or "little horse," but only part of a horse, perhaps its head, was implied by ancient writers. There was another Equuleus in southern skies, Equuleus Pictoris, the "painter's easel," but it is now called simply Pictor. Equuleus is a nickname for an easel, as well as a little horse.

To locate Capricornus, proceed south from Altair, in the direction indicated by its two companions, about 20° (two palms), looking for the 3rd-magnitude β, which has a 6th-magnitude companion about 3' of arc away. Before you get there, however, you will probably see the two alphas, 6' apart, an optical pair. Nearby is ν and a little to the west are the two ξ's as well. This asterism is very pretty, and one of the features of Capricorn, representing its head. The eastern part of Capricorn is marked by the pairs δ-γ and, a litte to the south, κ-ε. Capricorn is a boat-shape defined by a V of stars with ω at the point of the V, and a string of stars between β and δ at the top. Going south from β along the same line from Altair brings one first to a little triangle of stars, ρ, π and ο (near the ecliptic and Uranus), and then, about twice as far on, to the pair of stars ψ and ω. In the east, there is another V of stars converging on δ.

Once the Winter Solstice was in Capricorn, but no longer, due to precession. It still gives its name to the Tropic of Capricorn, at 23.5° south. This is the farthest south latitude at which the sun is directly overhead at some time during the year. Its northern counterpart is the Tropic of Cancer, though the Summer Solstice is now in Gemini. Incidentally, "tropic" refers to turning, where the sun turns around and moves north again.

Now look far to the north. Between Deneb and Polaris, a little closer to Deneb than Polaris and a bit to the east of the line, is the 2nd-magnitude Alderamin, α Cephei, the brightest star in the area. It is at the southeast corner of a quincunx-like figure, a rhombus with a star in the center. It will be the Pole Star in AD 7500. There are several stars in the southwest corner, forming an interesting asterism. Once you have identified this rhombic figure, look for γ towards Polaris, that seems to form the peak of a gable, so that it all looks like the front elevation of a church or house. Gamma will be Pole Star in AD 4000. This is Cepheus. The star in the far southwest corner, δ is a famous variable, giving its name to the class of variable stars called Cepheids. It was discovered to be variable in 1784, between magnitudes 3.6 and 4.3, with a period of 5d 8h 48m.

Cepheids have very regular periods from about 1 to 50 days, and the longer the period, the brighter the star. The absolute magnitude of the star can be inferred from its period. Since these stars are very bright, they can be seen at great distances, even in external galaxies. Comparing the observed magnitude with the absolute magnitude derived from the period, the distance of the star can be found. This was first worked out by Henrietta Leavitt in 1912, and has proved an excellent tool for discovering the scale of the universe. δ Cephei itself is very distant, about 1000 light-years.

The View From Sydney

Scorpius is still a magnificent sight west of the zenith. The eastern skies are dark and devoid of bright stars at this time. Alpha and Beta Centauri are high in the SW, with Crux beyond at 15° above the horizon. A triangle of stars of Carina, β, ε and ι, are just above the southern horizon. Canopus is in the SSE, only 2° in elevation. To the southeast, Achernar is at 36° elevation, the only bright star in a wide area. The Summer Triangle is in the north, with Altair at 45° elevation, Deneb and Vega at about 11°. The great square of Pegasus is rising in the NE. The September Triangle discussed above is just north of the zenith, and the constellations around it are all easily visible.


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Composed by J. B. Calvert
Created 20 February 2001
Last revised 19 May 2014