Thursday, June 20, 2019

King of the sky



Right now, the planet Jupiter rises in the east just about sunset. This means that Jupiter is "in opposition" and visible all night.

Jupiter is the most massive, and the largest planet in the solar system. All the other planets would easily fit into it's volume, with room to spare for a second set of solar system planets. We have not detected a solid surface on Jupiter yet; it is one of the two gas giant planets orbiting the sun (which one is the other?). It has the mass of over 300 Earths and around 1200 times the volume. with a diameter about 11 times that of Earth. It is a gas ball with a diameter of 143,000 km and consists of Hydrogen (about 90% and 9+% Helium, with traces of Methane and other elements). That's somewhat reminiscent of the Sun's composition; some people are calling Jupiter a "failed star".  However, in order to start the necessary nuclear fusion, Jupiter would have to have about ten times more mass. None-the-less, internal pressures are huge, enough to transmute Hydrogen and Helium into a liquid metallic state. At the centre, there may be an ice/rock core.

At opposition (i.e. now, June/July 2019) it is closest to Earth, since Sun, Earth, and Jupiter are essentially positioned in a straight line. That means that Jupiter is around 4 Astronomical Units away from us (1 AU = 150 million km). Therefore, the light coming to us from Jupiter takes around 35 minutes to get here. Add to that the distance of Jupiter to the sun, and you get about 45 more minutes. Since all solar planets shine by reflected sunlight, we see Jupiter illuminated by sunlight which left the sun around 80 minutes (1 hour and 20 minutes) ago.

Even though Jupiter is 5 times as far from the sun as Earth, and receives only 1/25th (square of distance law) of Earth's sunlight,  it is, after the Sun, the Moon, Venus, and the space station in orbit around Earth, the brightest object in the sky. That is due to its large diameter and relatively high reflectivity (albedo). It appears large enough from Earth to show some details in small telescopes, as well as the Galilean moons. Even binoculars will show the dance of the moons around Jupiter. Larger telescopes, i.e. 4", 5", 8" and larger will show many always changing details on Jupiter, including the great red spot, a local storm as large as our Earth, undergoing quite drastic changes at this time. Jupiter turns once in just under 10 hours; constantly showing new features on its surface. The prominent cloud bands (mostly methane, some ammonia, water ice, traces of hydrogen sulfide) move at hundreds of kilometers/hr., they are far more energetic than the worst storms on Earth.

The Galilean moons orbiting Jupiter show relatively fast changes in relative positions. These are easily followed even in binoculars. The moons themselves are very different from each other. In order of increasing distance from Jupiter they are Io, Europa, Ganymede, and Callisto. Io is the most volcanically active of any planets or moons in the solar system, Europa has a very smooth, icy surface, possibly about 50km thick, with a vast layer of liquid water suspected underneath; Ganymede is the largest moon in our solar system, and has a quite complex composition; Callisto is the third largest moon, after Ganymede, and Saturn's moon Titan. It has a heavily cratered surface. Only the largest of Jupiter's moons features are visible through earth-based telescopes, and a large telescope is needed even for that purpose.

Look up Wikipedia and other astronomical on-line sources to get more details for all Jupiter-related phenomena.

People have spent a lifetime exploring Jupiter's system, and now the latest orbiting probes are sending back very detailed data.

Jupiter is truly the King of the Solar System planets.








Wednesday, April 10, 2019

An easy target

While our winter nights in our area are cloudy for the most part, aside from the famous objects observable (the Orion nebula, say) we have some other beautiful, easily accessible star clusters available to be observed when the sky clears. One of these, an open cluster M35, in Gemini, can be seen through binoculars, and is almost overhead during that time of year, and can be found lower in the sky well into the spring, too. There is another star cluster located close to M 35, New General Catalogue (NGC) # 2158, much fainter.

M 35 is estimated to be about 2800 light years away, while NGC 2158 is about 5 times farther away. These two clusters are not related, their proximity is just a matter of perspective from our location in our galaxy. NGC 2158 can be observed through telescopes, while 10x50 binoculars resolve the brighter stars in M 35. This cluster is roughly 110 Million years old, pretty young, if we consider the age of the Universe (about 13.7 billion years). M 35 has a diameter of about 23 light years. We're getting closer to it at a speed of about 5 km/sec.



M35 (upper left hand quadrant) and NGC 2158 (lower right). I took this picture through a remote-control Slooh.com telescope


 Below is a map of M 35's location, a screen capture from Starry Night Pro by Simulation Curriculum:


You can also see the location of a few more objects, by no means all, which are easy to find in binoculars, and are really nice in 3 to 6 inch telescopes, at low power.

There are a number of "easy targets" in the sky at this time of year; I picked just one.






Friday, March 29, 2019

Visual aid



Many times, during our public astronomy nights, people have asked about buying a telescope. If this request comes from someone who has just looked through a telescope for the first time, I recommended to start with binoculars. This is the second step after having acquired at least a little familiarity with "naked eye" constellation, location of some of the major "targets", and the their seasonal visibility. I also explain the reason for the timing on the orbital position of the Earth in its orbit around the sun.

Having used telescopes for most of my life, I always use binoculars as helpers to locate objects I want to look at. I am talking here about finding them "manually", not using computerized telescopes. In many cases, people already own some binoculars, they've just never thought of using them to look at the sky. Most of these "found" binoculars are quite suitable for this kind of use.

For astronomical purposes, binoculars with larger front lenses are better. Astronomical objects, other than the Moon and the bright planets, tend to be quite faint; the larger the front lens, the more light you gather, that makes these faint targets easier to see. Personally, I use two pairs of binoculars with the magnifying power (power equals magnification) of 10 and one stabilized pair with the power of 15 (see fig. 1, left to right).

The OLYMPUS binoculars on the left, above, have 10 power and a front lens diameter of 42 mm (fig. 2), the Bushnell and stabilized Canon (fig. 3 and 4) have 50 mm diameter front lenses. Power and front lens diameter (in millimetres [mm]) are displayed somewhere on each pair of binoculars in the form of POWER x DIAMETER, i.e. 15x50 for the Canon binoculars. In addition, the field of view in degrees is also shown.


fig. 1

fig. 2

fig. 3

fig. 4


Generally, the larger binoculars become, the heavier they are. In this group of binoculars, above, the OLYMPUS pair is the lightest, while the Canon pair is the heaviest - it contains the stabilizing electronics, as well as a battery, in addition the internal prisms which make the image appear upright. All commonly available binoculars shown are, in a basic sense, refractor type telescopes, which will normally show an upside down image. Additional optical components are needed to turn the image around once more; that delivers an "upside down upside down" image. Galileo would have given his eye-teeth to have a telescope of the quality in today's refractors and their derivatives, i.e. binoculars.

I should also mention the effect of what is called the "exit pupil" of any optical telescope, including binoculars. The exit pupil can be seen by looking at the eyepiece (the lenses where you place your eye(s) to look through a telescope). Exit pupil diameter, usually stated in mm, should match the diameter of your eyes' pupils when you use binoculars. Exit pupils are the front lenses projected by the eyepieces, and therefore contain all the light entering the front lenses (see fig. 5). If your own eye pupils, which vary their diameter depending on the brightness at which you are looking is high, your pupils contract (become smaller). If your eye pupil is smaller than the exit pupils of your binoculars, you automatically discard some of the light which is contained in the binocular's exit pupils. This is no problem when you use your binoculars during the day, but for Astronomy, you usually look at very faint objects and you want to get all the light that you can catch in the objective lenses.

There is a simple way to calculate the diameter of telescope or binocular exit pupil diameter:

DIAMETER divided by POWER i.e.    50 divided by 10 = 5mm
      42 divided by 10 = 4.2mm
      50 divided by 15 = 3.3mm

fig. 5


When you are young, your eye pupils can expand to about 7mm in deep darkness. That's why 7 x 50 binoculars are used in the navy. Most sailors tend to be young, and can make use of all the light coming into the binoculars at night. As you get older, the maximum "dark adapted" pupil diameter tends to get smaller. For instance, at age 50 you may only have a 5mm "dark adapted" pupil diameter. I'm way past that age, so my pupil diameter may possibly expand to less than that. Again, if your pupils match the diameter of exit pupil, you see all the light that your telescope or binoculars can catch. Any pair of binoculars worth their salt will also show you the larger craters on the Moon, the Galilean moons of Jupiter, brighter open and globular star clusters, movements of the planets and other interesting astronomical wonders. Much of this information is found in the RASC's Observers Handbook (free if you are a member).

As a couple of examples, there are detailed descriptions of the technical aspects of binoculars by Dr. Roy Bishop contained in the RASC's Observers Handbook, starting on page 60 in the 2019 edition. Wikipedia also contains information about binoculars.

Think of how much more light the objective lenses of telescopes and binoculars can intercept than your own pupils (50mm versus 5mm, say). It is the ratio of the disk area of a 50mm objective lens and 5 mm exit pupil you need to consider.

You can see what a great visual aid binoculars can be when using them to look at the sky at night.


Thursday, January 24, 2019

Serendipitous Eclipse

It happens very seldom that the weather gives us a break exactly when we astronomical observers want one. Usually it's the reverse: the weather is reasonable just before an interesting event. Then,  just a few hours before event time the weather clouds up, or rain starts, or snow, or any other condition which prevents us from observing is going to occur.

This time, we got a lucky break. The weather cleared up just a few hours before the recent total lunar eclipse. Our granddaughters, who are also members of the RASC (as I am), took some nice pictures of the eclipse (I set up a telescope on the rear porch at home and just observed by eye, binoculars, and the telescope). I downloaded only one image from Slooh. com the remotely-accessed organization.

Here is one of Marina's pictures.

Image may contain: night



Here is my only image, taken with a remote-control telescope at Slooh.com:



I'm always fascinated by the media's statements when they are hyping a completely normal astronomical event which repeats itself often with descriptions like "Blue Moon, Harvest Moon, Wolf Moon, Super Moon", and other mystical names.

We will have no more total lunar eclipses this year. A partial lunar eclipse this year will occur on July 16. The next total lunar eclipse will occur on May 26, 2021. On November 19, 1921 will be a partial lunar eclipse. Another total lunar eclipse occurs on May 16, 2022. In between occur several penumbral lunar eclipses, on Jan 10, 2020, June 05, 2020, July 05, 2020, Nov 30, 2020 all usually almost invisible, because the brightness of the full moon varies very little. Penumbral lunar eclipses are caused by the Moon just missing the Earth's shadow.

The Moon's orbital plane differs from the Earth's orbital plane around the Sun by about 5 degrees. There are two crossover points, the ascending and descending nodes. Both solar and lunar eclipses (partial and total) can occur only when these nodes are "in line" with the Sun, and the Moon is very near, or at one of them. That also means the both types of eclipses occur at either "new Moon" (solar) or "full" Moon (lunar). These nodes slowly move around the plane of the Earth's orbit, giving rise to various series of "eclipse cycles" which repeat over hundreds of years.

You can let your imagination play by thinking about what these various types of eclipses would look like if you found yourself on the Moon...

The total lunar eclipse on January 20 was indeed serendipitous.