The Night Sky This Month: September 2021

Possible orbital path of Planet X

The notion of an unknown planet being present somewhere in the solar system does not have a universal appeal among astronomers. The gravitational effects of such a planet, however, remains the most plausible explanation for the (as yet) unexplained behavior of many Kuiper Belt objects.

Therefore, after some years of research into the apparent displacements of some Kuiper Belt objects, Caltech astronomers Mike Brown and Konstantin Batygin, two prominent proponents of Planet X, as the elusive ninth planet had been dubbed, have produced a kind of map that shows the most probable locations or orbital path(s) of Planet X. In the image above, the light blue band represents our sky, while the wavy black line that follows the blue band represents the ecliptic. The red part in the image represents the most likely deviations from the ecliptic that Planet X might follow.

However, the Caltech astronomers are at some pains to point out that their map only highlights the most probable locations Planet X should be in (based on observed positions of many Kuiper Belt objects) as opposed to the path the planet is actually following through the solar system. The problem is that since the putative planet will likely have an orbital distance of between about 300 and 380 AU, and an orbital inclination of about 16 degrees, the planet could be at almost any point within the suggested map at any given time. Moreover, if one adds to this immense search area the facts that Planet X will likely have a magnitude of as low +22, then it becomes clear that finding the elusive planet will be no easy matter- if it is ever found.

The Moon Phases in September 2021

New Moon First Quarter Full Moon Last Quarter
September 7th September 13th September 20th September 29th

NOTE: This year, the September Equinox occurs at 19:11 UTC on September 22nd. At this time, the Sun will be directly above the equator, producing equal amounts of light and darkness throughout the world. This date also marks the beginning of autumn in the Northern hemisphere and the start of spring in the Southern hemisphere.

The Planets in September 2021

Planetary viewing remains poor for observers north of latitude 51.51N, and this will continue to be the case for the next several months. Nonetheless, some details of the state of the planets are listed below-

Mercury recently passed behind the Sun, and will not be visible for most northern observers, seeing that the little planet reaches its highest elevation during daylight hours. By dusk, the planet will be about 1 degree below the horizon as September starts, and about 8 degrees below the horizon at dusk at the end of the month.

Venus is now becoming visible in the early evening as it approaches a point of greatest eastern elongation from the Sun. Sadly, it will not rise above about 5 degrees above the horizon at any point during the month.

Mars will not be visible to most northern observers, since it is now within 12 degrees of the Sun. By the end of September, the Red planet will be within about 2 degrees of the Sun.

Jupiter is approaching a point of opposition, and it will become visible for most northern observers from about 20:26 (BST) to about 04:00 (BST). However, the King of the planets will only rise to an elevation of 24 degrees above the southern horizon at about 00:13 (BST), which could make it difficult to obtain clear views of the planet.

Saturn has just passed a point of opposition, and it is now becoming visible as an early evening object from about 20:25 (BST). Note, though, that the planet will not rise above about 19 degrees above the southern horizon by 23:00 (BST), and that it will become inaccessible by about 01:49 (BST).

– Uranus is currently visible as an early morning object from about 21:42 (BST), and it will reach an elevation of about 54 degrees above the southern horizon as the month starts. By month’s end, the planet’s maximum elevation will reduce to about 43 degrees before dawn, which then occurs at around 05:42 (BST).

– Neptune is now approaching a point of opposition, and it is therefore visible as an early morning object. Nevertheless, the planet will only rise to an elevation of about 34 degrees above the southern horizon before becoming inaccessible at an elevation of 22 degrees, which occurs at about 02:51 (BST).

Meteor Showers in September 2021

The Epsilon Perseids Shower is expected to peak just before dawn on the 9th of the month when its radiant is at its highest point above the horizon. However, the Epsilon Perseids is not known for its splendor, delivering as it does, no more than about 5 meteors per hour during the peak period.

Moreover, this low rate assumes both a perfectly dark sky and a radiant that is almost directly overhead. Therefore, since the radiant will be an elevation of about 78 degrees above the horizon as seen from London, observers can expect to see no more than 4 meteors per hour during the peak hours.

In practice, this low rate can be difficult to distinguish from background meteor activity, especially given that meteors from this shower can appear to come from any point in the sky.

Deep Sky Objects to Look for in September 2021

This month, we highlight a few easy targets for binoculars– one in the constellation Cygnus, one in the constellation Perseus, and one in the constellation Camelopardalis, a little-known constellation that has no stars brighter than magnitude +4.0. Below are some details, but note that while we recommend the use of image-stabilised 10 × 50 instruments with 3-degree fields of view to view the objects listed below, smaller instruments with smaller fields of view will also yield good, albeit truncated views-

 The North America Nebula (NGC 7000, Caldwell 20)

North America Nebula
Image credit: Public Domain

Located at a distance of 2,590 light away in the constellation Cygnus, this iconic nebula appears to consist of two separate areas of nebulosity; the North America Nebula to the upper left, and the smaller Pelican Nebula to the lower right. However, the two parts are, in fact, one massive nebula, and the illusion of separation is caused by a dark nebula that is superimposed over the brighter part, thus “splitting” the nebula into two parts.

In terms of dimensions, the nebula, which is more properly referred to as an HII region, stretches 140 light years across its widest part and 90 light years across its narrowest part. While it was long thought that the nebula is being ionized by the bright star Deneb, the ionizing star is, in fact, a star designated J205551.3+435225, a massive young star with a surface temperature of at least 40,000K.

Kemble’s Cascade

Kemble’s Cascade
Image credit: Steve Tonkin

Although the constellation Camelopardalis is rather inconspicuous in the northern sky even under the best viewing conditions, the sight of this cascade of stars is certainly worth the trouble of hunting it down.

Since this constellation does not have any particularly bright stars to use as markers, it helps to use the stars Caph (Beta Cassiopeiae) and Segin in neighboring Cassiopeia to star-hop. Thus, use a line between the two stars, and double it into Camelopardalis; at this point, start panning straight upwards until the line of 15 magnitude .+8.0 stars appear in the field of view. You will know you have the right line of stars when you notice a single, bright, magnitude +5.0 star marking the midpoint in the line of stars.

While the view shows the cascade of stars at an angle, this line of stars cascades almost vertically downwards during September, seemingly plunging into NGC 1502, a fairly large open cluster that forms a kind of “splash pool” at the foot of the cascade.

The Alpha Persei Moving Cluster (Melotte 20, Collinder 39)

Alpha Persei Moving Cluster
Image credit: Martin Gembec

Located about 570 light years away in the constellation Perseus, the large cluster, or more properly, an association of 0 and B stars, is relatively easy to find to the southward of Mirphak (Alpha Persei), the brightest star in the constellation. It is shown here as the over-exposed star near the centre of the frame.

While all the stars in the inner region of the cluster share the same proper motion across the sky, their most attractive aspect is the fact that they seem to “sparkle like diamonds on black velvet”, which is how their discover described them in his notes.

As open clusters go, the Persei Moving Cluster is not particularly big; its tidal radius spans only about 70 to 80 light years, and its half-mass radius is only about 18 light years across. However, despite its relatively modest dimensions, the cluster shows clear evidence of a process of mass segregation, with the most massive stars having started migrating towards the core, while stars with lower masses are drifting outward towards the cluster’s fringes.

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