August Skies

Perseid meteor shower 2018:

What is the Perseid meteor shower?

The Perseid meteor shower is one of the brightest and most active meteor showers in the year. It occurs in July and August in the Northern hemisphere making it a summer shower and one of the most popular. The peak occurs around the 12^th to 13^th August. Showers begin July 17^th and last through to August 24^th.

In dark, rural locations visible Perseid rates can reach 50-100 meteors per hour. The meteors are relatively fast moving - entering the Earth's atmosphere at speeds of approximately 37.5 miles per second - but are known to produce trails which linger for a while.

The Perseid meteor shower comes from the debris of Comet Swift-Tuttle.     The Perseids were the first meteor shower to be connected to a comet when astronomer Giovanni Schiaparelli noted the similarity between their orbit and the Comet Swift-Tuttle observed in 1862.

In 1835, Adolphe Quetelet identified the shower as emanating from the constellation Perseus. In 1866, after the perihelion passage of Swift-Tuttle in 1862, the Italian astronomer Giovanni Virginio Schiaparelli discovered the link between meteor showers and comets. The finding is contained in an exchange of letters with Angelo Secchi.

Figure 1 below show the 2018 highlights of the Perseids meteor shower.

A meteor shower being a group of meteors that appear to come from one point on the celestial sphere. Each shower is named after the constellation in which this radiating point or radiant is situated (figure 1 and 2). The shower has its origin in a meteor stream, the debris left by a comet in its path, when the Earth moves through it. Since the paths of comets are narrow and in a defined position a given distance from the Sun, if the Earth once cuts across the orbit of a comet, it will do so again once a year, thus allowing dates to be given when a shower would be expected.

Figure 3 shows a Perseid meteor streaking across the night sky. A meteor being a ‘shooting star’, the streak of light seen on a clear night when a meteoroid burns out due to friction when it passes through the Earth’s upper atmosphere.

Figure 1 :-

Figure 2 :-

Figure 3 :-

Other meteor observations during August is the Capricornid meteor shower (figure 4/5), this will reach its peak of activity on 2^nd August. Shooting stars which are part of this shower should be observable between 15^th July and 20^th August. The observation rate is dependent on viewing conditions and the main point of the shower being directly overhead.

Figure 4 :-

Figure 5 :-

The meteor shower was created about 3,500 to 5,000 years ago, when about half of the parent body (comet 169P/NEAT (Near Earth Asteroid Tracking)) disintegrated and fell into dust. There could be peak rates of 2 to 5 per hour, sometimes having outbursts of bright flaring meteors with rates up to 5 to 9/h. The parent body responsible was discovered March 15^th 2002.

The meteor shower was discovered by Hungarian astronomer Miklos von Konkoly-Thege in 1871.This shower has infrequent but relatively bright meteors, with some fireballs.

Only the Southern Delta Aquarids (SDA, peak July 28-29) and Alpha Capricornids (CAP, peak Aug 2–3), with their higher rates, are listed separately by the IMO (International Meteor Organisation). Older listings sometimes include a northern Delta Aquarid shower peaking around Aug 7, with a radiant near the Aquarius–Pisces border.

On 6^th August shooting stars emanating from (Tau) Aquarids will be at its intensity of display and their visibility lasts between July and August. When they burn up in the Earth’s atmosphere this takes place between heights of 70km (43.75 miles) and 100km (62.5 miles). The meteor showers are of pebble size as they burn up. The parent body responsible for this meteor shower has not been identified. If visible there should be upto 8 per hour.

The moon (figure 6) at perigee (closest point) (figure 7) happens on August 10^th. The distance from Earth varies because its orbit is not perfectly circular, it is instead slightly oval-shaped, tracing out a path called an ellipse.   

Figure 6 :- Figure 7 :-

On 11^th August the Moon passes in front of the Sun, creating a partial solar eclipse. The alignment will not be very exact and nowhere on Earth will see a total eclipse.

The path of the eclipse will pass through various countries, including:

From the UK, the Sun will be eclipsed to a maximum of 2%. Never attempt to view the Sun directly with the eyes or through optical instruments as this can cause eye damage or even blindness within seconds. Specialist astronomy suppliers sell approved filters for viewing the Sun.                                                If unsure seek specialist advice.

On 14^th August Messier 15 (M15), also known as the Great Pegasus Cluster (figure 8/9), is a globular cluster located in the northern constellation Pegasus. The clusters distance is 33,600 light years from Earth. It has the designation NGC (New General Catalogue) 7078. Star clusters are groups of stars sufficiently close to each other for them to be physically associated. These stars are formed together from the same cloud of interstellar gas and have approximately the same age, and initial chemical composition.

Figure 8 :-

To the eye M15 is very faint, but can be viewed through a telescope or binoculars. It can be detected between Pegasus Equuleus and Aquarius, ranging from South East horizon to South West horizon.

On 15^th August globular cluster M2 (NGC 7089) in Aquarius (figure 9) will be well placed for observation. It will reach its highest point in the sky about midnight local time. Visible all night. Visible at 22:15 (BST) as the dusk sky fades, 25° above your south-eastern horizon. It will be lost to dawn twilight at 04:28, 24° above south-western horizon. M2 is quite faint, and not visible to the eye, it can be viewed through a pair of binoculars or telescope.

Figure 9 :-

The α–Cygnid meteor shower (figure 10) will be active on 21 August. Some shooting stars associated with the shower are expected to be visible each night from July to August. The radiant of the shower appears 68° above the eastern horizon at midnight. There should be approximately 4 meteors per hour, since the radiant will be high in the sky, maximising the chance of seeing meteors.                                                                                                                                   The Moon will be 10 days old at the time of peak activity, and being so close to full Moon will severely limit the observations that will be possible. To see the most meteors, the best place to look is not directly at the radiant itself, but at any dark patch of sky which is around 30–40° away from it. It is around this area from the radiant that meteors will show reasonably long trails without being too spread out.

Figure 10 :-

To date, the parent body responsible for creating the α–Cygnid shower has not been identified.

To aide viewing of the various objects on display in the sky and beyond use the Stellarium which can be found on the web page.

Planets

Venus (figure 11), can be seen low in the west after nightfall sinking towards the horizon as the month advances. During August, its illuminated phase thins, but, at the same time, the angular diameter of its disk increases. The surface area reflecting the Sun's light increases and therefore brightness increases.

Venus moves towards Spica in Virgo as August progresses and ends the month one degree below the star. They are then only ~10 degrees above the Western horizon after sunset.

Figure 11 :-

Venus showing some cloud structure

Jupiter (figure 12),can be found in the southwest after sunset at the beginning of the month. Jupiter's equatorial bands, sometimes the Great Red Spot and up to four of its Galilean moons will be visible using a small telescope. Unfortunately travelling slowly westwards in Libra during the month, Jupiter heads towards the southern part of the ecliptic and will have an elevation of ~15 degrees after sunset. Atmospheric dispersion will hinder the view. Serious enthusiasts may consider using an Atmospheric Dispersion Corrector to counteract its effects.

Figure 12 :- A Cassini image of Jupiter, NASA

Saturn (figure 13), was at opposition on the 27th of June and so will be visible in the south at an elevation of ~15 degrees after sunset at the beginning of August.

Its brightness reduces as the month progresses. The rings were at their widest some months ago and are still, at 26 degrees to the line of sight, well open and spanning ~2.5 times the size of Saturn's globe. Saturn, positioned in Sagittarius, is close to the top most star of the 'teapot' slowly moving in retrograde to within a few degrees of M8, the Lagoon Nebula, and M20, the Trifid Nebula.

Unfortunately atmospheric dispersion will spoil the view and, again, it might be worth using Atmospheric Dispersion Corrector to counteract its effects.

Figure 13 :- The planet Saturn. Cassini - NASA

Mercury (figure 14), Passing between Earth and Sun (inferior conjunction) August 9th, becomes visible after the 20th before reaching greatest elongation east of the Sun on August 26th.

Figure 14 :- Messenger image of Mercury Nasa

Mars (figure 15), in retrograde motion westwards in Capricornus made its closest approach to Earth since 2003 on July 30th/31st. Mars starts the August rising after sunset shining at its peak magnitude but this falls by end of August. With a small telescope it should (see below) be possible to spot details, such as Syrtis Major, on the surface. From the UK, it will only reach an elevation of ~14° when due south and so, unfortunately, the atmosphere will hinder the view. A good reason to use an Atmospheric Dispersion corrector!

In July a dust storm obscured much of the surface, if this continues viewing will be greatly reduced.

Figure 15 :-

A Hubble Space Telescope image of Mars.
Jim Bell et al. AURA / STScI / NASA

Mark R Smith FRAS

Physicist

Nuclear Fusion & Astrophysics