Mercury and Venus

Fig 1: Venus imaged February 19 2006. Exp. 1/1000 sec. ISO 350, 175mm Maksutov 212x. (See text for planetary data.) Click the image for an enlargement.

Fig. 2 Mercury imaged February 20 2006. Exp. 1/180 sec. ISO 350, 175mm Maksutov 212x.(See text for planetary data.)

Mercury is quite readily visible to the unaided eye in the evening sky from about 18h 00m. until shortly before setting (19h 15m on the 21st Feb. see notes below).

Mercury may be observed for the next week in the daytime sky with quite small telescopes (85 mm aperture). It is now fading quite rapidly. It may best be seen at southing (meridian passage) which occurs at 13h 30m on Feb. 24 (altitude 29.8 degrees).

Venus is close to maximum brilliance and may be picked out with the naked eye close to southing which occurs at 09h 33m on Feb. 24 (altitude 14.2 degrees).

There have been conditions of good seeing here on Rousay for the past two days. Even at low altitudes with Mercury well east of the meridian it has been possible to use powers up to 220x on the 170mm Maksutov.

Two images obtained for Venus and Mercury make interesting comparisons. Note: the shorter exposure for Venus is responsible for the darker sky background. At the time of the photographs data as follows: Mercury (Feb. 20 11h 27m) dia. 6.48", phase 65%, mag. -0.8: Venus (Feb. 19 08h 56m) dia. 39.32", phase 27.1%, mag. -4.6.

JV
22/02/06

Mercury in the evening sky February/March 2006 - Exposing some myths.

The sky looking SW on March 1st 2006 at 18h 30m UT The phase of the Moon will be a slender 5%. Click the image for an enlargement.

There will be a good opportunity to observe Mercury in the evening sky from mid-February through to the first few days of March.

Those with a clear SW horizon may have the opportunity to see the planet from around the 10th when it sets (17h 15m) almost 1 hour after the Sun.

During the course of the following two weeks Mercury's brightness will diminish slowly from magnitude -1.2 (a little fainter than Sirius) to about half this at magnitude -0.4.

Thereafter it fades at an ever accelerating rate, halving in brightness in just two days from March 4 to March 6.

The last opportunity to see Mercury with the unaided eye may be on either March 2 or 3. Despite setting 85 minutes after the Sun, Mercury will be difficult to locate in the strong twilight at magnitude 1.3.

The rapid fall off in brightness is a factor of phase and the reflected angle of the Sun's light towards the Earth. Despite both being inferior planets (with orbits inside the Earth's) the apparent brightnesses of Mercury and Venus behave quite differently.

Venus is brightest close to maximum eliongations when the phase is around 50%. In the case of Mercury greatest brilliance occurs when the planet is near full phase and therefore close to the Sun as seen in the sky.

The differences are due to the marked contrast in the physical nature of the two planets; Venus heavily cloud-laden with a high albedo, whilst Mercury is to all intents and purposes without an atmosphere and with a low albedo (similar to the Moon).

Naked eye observations of Mercury necessarily have to take place when the planet is at relatively low altitudes in twilight. This is not a good time to be observing the planet telescopically. Until the use of space probes we had little idea what the surface of the planet looked like. Even in large telescope with Mercury high in the sky during daylight little can be seen beyond vague smudges. (Remember at around 50% Mercury has an apparent diameter of around 6 arc seconds, or a little over 1/3 that of Mars at its recent opposition.)

Despite being a disappointing study in the telescope it is not difficult to locate Mercury in the day sky. The difficulties have often been overstated. Even as recently as 2003 we have the authors of Exploring Mercury - The Iron Planet (Strom & Sprague) stating (page 5): "... Mercury is often difficult to find. This was especially true in the past before telescopes benefited from computerized finding and tracking systems."

This is nonsense, of course. Equatorial telescopes equipped with circles and drives have been around for over 200 years. It is no more difficult to locate Mercury in a clear, daylight sky with a good equatorial telescope than to find faint double stars way beyond naked eye visibility at night .

There is another myth that is worth exposing and that is the quality of the seeing in daylight. Contrary to all logic, when the Sun's presence in the sky should make matters difficult, seeing can at certain times of day be as good as, if not better than, night seeing. During my own work as a double star observer I very often found strong twilight to offer better seeing than through the hours of darkness.

In the case of Mercury I have had many opportunities to observe the planet close to the meridian with good instruments (refractors of 250 mm aperture) at elongations of 9 arc degrees with powers of up to x600.

My first recorded observations of Mercury were in the early 1950s when I was still at school. I note from my diary dated 22 March 1952: "At 19h 00m observed Mercury with 60x50 refractor. The crescent phase was readily seen despite the planet being at an altitude of 8.5 degrees."

JV
06/02/06