Mars and a little matter of scale

Fig 1 Moon imaged November 5 2004 04h 45m UT. 175 mm x 60 Maksutov, 1/300 sec ISO 100.

Fig 2 Moon imaged November 5 2004 04h 45m UT. 175 mm x 212 Maksutov, 1/90 sec ISO 100

Not having seen Mars for several months, and having heard a number of rumours as to what might be happening to the red planet, I was relieved to see it in the telescope this morning looking its old self!

The internet is a wonderful place.

For example, somebody started a tale about Mars appearing in the sky latter this year as large as the full Moon. At its closest to Earth, on October 31, 2005, Mars will appear a little over 20 arc seconds in diameter. That is 1/90th the apparent diameter of the Moon.

It is all a question of scale.

I am sometimes asked if we could see an island the size of Rousay (about 5 miles by 4 miles) on the Moon using a small telescope. How small is small?

A good 100mm refractor would be quite capable of revealing craters of 6 miles diameter or less.

Fig 1 shows the Moon photographed in the early hours of November 5, 2004. The circled area indicates the region around Rupes Recta. Fig 2 is a large scale of the same region. The crater to the left is Birt (10 miles diameter) with Birt A (4 miles diameter) intruding into the wall of the larger crater.

To put Mars into the lunar perspective, consider again the planet on October 31.

A feature on the Moon's surface would have to be 22 miles in diameter to subtend the same angle (20") as Mars. So, rather than appearing as large as a full Moon, we must think in terms of something about twice the size of crater Birt.

And as we have indicated, a telescope of around 60mm aperture would be required to see this feature.

Fig 3 Mars imaged 06h 02m UT August 3 2005. 160 mm OG x 175, 1/90 sec ISO 100.

Fig 4 Mars imaged (mono) 06h 04m UT. Data as fig 3.

Yet another way of looking at the problem is to place a one pound coin (diameter 22mm) at a distance so as to subtend the same angle as Mars on October 31. The result is 225 metres. Try it.

If, on the other hand, Mars were to be viewed at the same distance from Earth as the Moon is from us, it would appear twice as large as the Moon.

JV

Mars - August 3

Mars crossed the local meridian in Orkney this morning at 05h 33m UT.

There happened to be a clear break in the clouds allowing good views of the planet in the telescope. (Note, the Sun by then was well above the eastern horizon.)

The phase of Mars at 84% (diameter 11.45") was the most noticeable feature. The extensive south polar cap was quite plain at a power of x100. This feature gives the illusion of being detached from the disc.

The seeing was sufficiently steady to enable a power of x275 to be used. Dark surface markings were also readily seen but imaging tends to be disappointing with ordinary digital cameras. (See Figs 3 & 4.)

Note, using JV's one pound coin analogy, Mars was 398 metres distant this morning!

JM 03/08/05

Paths of the Superior Planets and astronomical nomenclature

Those planets of the Solar System with orbits outside that of the Earth are sometimes referred to as Superior Planets. Venus and Mercury both orbit the Sun within the Earth's orbit and are called Inferior Planets.

The use of the words superior and inferior in astronomy, like other familiar terms, may sometimes appear inappropriate if not downright confusing.

Another example is the use of the word magnitude to denote the brightness of astronomical bodies and not their size, as the word might suggest.

The coordinates Right Ascension and Declination appear quaint to some!

The motions of the planets against the background of stars, as seen from the Earth, have exercised the minds of astronomers for thousands of years. In fact, until comparatively recently, astronomy consisted mostly of observing the motions of bodies and making deductions therefrom .

Dia 1: The path of Mars throughout the rest of 2005 and into the first few days of 2006. Note the "loop" takes place almost entirely within the constellation Aries.

The first use of the optical telescope for astronomical purposes by Galileo in 1609 can be said to herald the study of astrophysics. Even so, astronomy remained a passive pursuit in the sense that one had to depend upon observation and had very few options to conduct actual experiments.

A great deal has changed with the advent of space research. We can now put many of our theories to the test; it is a tribute to scientific knowledge and mankind's ingenuity that such remarkable fetes as the recent space mission to Saturn have become a reality.

But sending sophisticated apparatus into space is an expensive as well as a dangerous exercise. Most of us have to content ourselves with observing from the Earth's surface. And with ever increasing atmospheric pollution this is becoming more difficult to carry out.

The Retrograde Loop

When we plot the course of a superior planet against the star background we see that the regular forward motion - west to east - is interrupted for a time by a period of retrograde motion - east to west. (Do not confuse this with the diurnal motion east to west of all celestial objects relative to the horizon seen from the northern hemisphere.)

The so-called retrograde loop will be demonstrated most noticeably by Mars from September 26 to December 6, 2005. (Dia 1) Halfway between these two dates Mars will be in opposition (November 7).

Strictly speaking in the case of Mars this is more a flattened "S" than a loop.

The width of this "loop" this year for Mars is 15.5 arc degrees. For Jupiter it averages about 10 arc degrees and for Saturn 7 arc degrees. The phenomenon occurs either side of opposition and so in the case of Jupiter, Saturn and the other superior planets it is an annual event.

Seen from Orkney, Mars rises a little north of east at 23 h UT (midnight BST) on July 20. At magnitude -0.3 it will be unmistakable. By the end of August, Mars will rise at 20h 45m UT and will have more than doubled in brightness.

Compared to the previous opposition in 2003, Mars will be well placed for northern observers in 2005. A more detailed account of the coming opposition will appear in these notes closer to the event.

JV 18/07/05