Thursday, 19 May 2011

The Atmosphere and Observing - A guide to Astronomical seeing.

An observer, be they at a mountain top observatory, or in their own back yard must, at all times contend with the Earth’s atmosphere. It is a notoriously unpredictable and limiting factor in obtaining fine views of the Planets, and close binary stars. Many often comment, especially here in the UK that seeing is all too often mediocre on most nights, but what are the factors that contribute to this?. Are there ways and signs, which indicate whether the atmosphere, will be stable or turbulent on a given night?.

What is “seeing”?
So what exactly is atmospheric seeing? - it is high frequency temperature fluctuations of the atmosphere, and the mixing of air “parcels” of different temperatures/densities. This behaviour of the atmosphere is seen at the eyepiece as a blurred, moving, or scintillating image. There are roughly 3 main areas where Atmospheric turbulence occurs. Near ground seeing (0 – 100metres or so.) central troposphere (100m – 2km), and High troposphere (6-12km.) Each area exhibits different characteristics, which are explained in more detail below. 

Beware of the Jetstream

  Clouds along a jet stream over Canada.

Jetstream can influence your "seeing" 

3. High Altitude effects.
Effects at this altitude are caused by fast moving “rivers” of air know as Jet streams. Wind shears at around the 200-300mb altitude level can cause images to appear stable, but very fuzzy, and devoid of fine detail. There isn’t anything the observer can do to prevent these effects, but forecasts are available, to help predict weather a Jet stream is present over your area. Areas of the Northern hemisphere most affected by the Polar jet stream are the Central US, Canada, North Africa, and Northern Japan. The Jet stream’s position varies with the seasons, tending to move further South during the winter and spring months.

Source: Wikipedia > Jetstream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude).[1] The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7–12 km (23,000–39,000 ft) above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km (33,000–52,000 ft).

Total Lunar Eclipse of June 15 - Also in Bloemfontein

Total Lunar Eclipse of June 15

The first lunar eclipse of 2011 occurs at the Moon's ascending node in southern Ophiuchus about 7° west of the Lagoon Nebula (M8). The Moon passes deeply through Earth's umbral shadow during this rather long event. The total phase itself lasts 100 minutes. The last eclipse to exceed this duration was in July 2000. The Moon's contact times with Earth's umbral and penumbral shadows are listed below.
   Penumbral Eclipse Begins:  17:24:34 UT
   Partial Eclipse Begins:    18:22:56 UT
   Total Eclipse Begins:      19:22:30 UT
   Greatest Eclipse:          20:12:37 UT
   Total Eclipse Ends:        21:02:42 UT
   Partial Eclipse Ends:      22:02:15 UT
   Penumbral Eclipse Ends:    23:00:45 UT

At the instant of greatest eclipse [5] the umbral eclipse magnitude [6] will reach 1.6998 as the Moon's centre passes within 5.3 arc-minutes of the shadow axis. The Moon's southern limb will lay 54.2 arc-minutes from the edge of the umbra while the northern limb will lay 22.3 arc-minutes from the umbra's edge. Thus, the northern regions of the Moon will probably appear brighter than the southern regions that lie deeper in the shadow. Since the Moon samples a large range of umbral depths during totality, its appearance will change dramatically with time. It is difficult to predict the exact brightness distribution in the umbra so observers are encouraged to estimate the Danjon value at different times during totality (see Danjon Scale of Lunar Eclipse Brightness). Note that it may also be necessary to assign different Danjon values to different portions of the Moon (i.e. - north vs. south).
Nearly 30 years ago (1982 Jul 06), the author watched another total lunar eclipse with the Moon in the same part of the sky. I was amazed at how brilliantly the summer Milky Way glowed since it was all but invisible during the partial phases. Observers will have a similar opportunity during June's eclipse. In this case, the totally eclipsed Moon will lie in southern Ophiuchus just 8° northwest of the brightest Sagittarian star clouds. The summer constellations are well placed for viewing so a number of bright stars can be used for magnitude comparisons with the totally eclipsed Moon.
Antares (mv = +0.92v) is 15° to the west, Shaula (mv = +1.63) is 14° south, Epsilon Sgr (mv = +1.85) is 15° southeast, Arcturus (mv = -0.05) stands 55° to the northwest, and Altair (mv = +0.77) is 46° northeast of the Moon.
Figure 3 shows the path of the Moon through the penumbra and umbra as well as a map of Earth showing the regions of eclipse visibility. The entire event will be seen from the eastern half of Africa, the Middle East, central Asia and western Australia. Observers throughout Europe will miss the early stages of the eclipse because they occur before moonrise. Fortunately, totality will be seen throughout the continent except for northern Scotland and northern Scandinavia. Eastern Asia, eastern Australia, and New Zealand will miss the last stages of eclipse because they occur after moonset. Again, the total phase will be seen from most of these regions. Even observers in eastern Brazil, Uruguay and Argentina will witness totality. However, none of the eclipse will be visible from North America. At mid-eclipse, the Moon is near the zenith for observers from Reunion and Mauritius.
Table 3 lists predicted umbral immersion and emersion times for 20 well-defined lunar craters. The timing of craters is useful in determining the atmospheric enlargement of Earth's shadow (see Crater Timings During Lunar Eclipses).
The June 15 total lunar eclipse is the 34th member of Saros 130, a series of 71 eclipses occurring in the following order: 8 penumbral, 20 partial, 14 total, 22 partial, and 7 penumbral lunar eclipses (Espenak and Meeus, 2009a) spanning 1262 years. Complete details for Saros 130 can be found at:

Tuesday, 17 May 2011

ISS & Endeavour visible from Bloemfontein

Watch out for the International Space Station and the space shuttle Endeavour passing over this evening (Tuesday).  Not sure how far apart they'll be - could be a minute or two - but they should both look like bright stars moving upwards from the north-western horizon at 6:57pm.  Both will "disappear" about halfway up the sky as they pass into the shadow of the Earth.

Endeavour launched Monday, and will dock with the ISS just after midday Wednesday.  The "crew hatch opening" at around 2:30pm (and other mission events) can be watched live at
This is the last flight of Endeavour.

For sightings of the Space Station from Southern Africa later this week, go to  click on "In the Sky" and then "Satellites", and follow the instructions

Claire Flanagan
Wits Planetarium
086-521-4273 (fax)

Info for Bloemfontein from Heavens Above

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Observer's location: Bloemfontein, 29.1330°S, 26.2000°E
Local time zone: Universal Coordinated Time -2 (UTC + 2:00)
Orbit: 343 x 346 km, 51.6° (Epoch May 16)

Date Mag Starts Max. altitude Ends
Time Alt. Az. Time Alt. Az. Time Alt. Az.
17 May -1.8 18:58:03 10 NNW 18:59:32 23 N 18:59:32 23 N
18 May -1.2 19:21:31 10 WNW 19:22:57 24 WNW 19:22:57 24 WNW
19 May -2.8 18:10:11 10 NNW 18:12:52 34 NE 18:14:47 16 ESE
20 May -2.6 18:33:35 10 WNW 18:36:22 42 SW 18:37:56 21 SSE
21 May -0.7 18:58:46 10 WSW 19:00:06 12 SW 19:00:56 11 SSW
22 May -2.6 17:45:30 10 WNW 17:48:17 42 SW 17:51:06 10 SSE
23 May -0.7 18:10:36 10 WSW 18:11:56 12 SW 18:13:15 10 S