Mercury/Regulus conjunction - update Sep 9

Last night (8/9 September) was exceptionally clear - a one in a thousand quality in my experience of Orkney over a period of 35 years.

Mercury rose at 03h 33.5m (GMT) into a perfect sky (contrast Tuesday when contrails were scoring the eastern horizon rendering observation impossible).

By 04h both Mercury and Regulus could be seen with the unaided eye. However, increasing twilight made Regulus a difficult object thereafter and by 04h 10m a binocular was necessary to show the star.

Mercury may be observed comfortably with an aperture of 100mm in full daylight. Regulus is also just visible with this aperture in a clear sky. With a 150mm aperture telescope working at x100 both objects are easy. During observable altitudes throughout today (Thursday), Mercury will be seen to close on Regulus from a separation of around 60' (or one arc degree) at 08h to half this value by 15h.

Fortuitously, Mercury and Regulus will be at their closest tomorrow morning at approximately 05h GMT (See below and archive.)

JV, Rousay, 09/09/04

Mercury update - Monday 6 September 2004

Following a fine night, Mercury rose into an almost perfectly clear sky at 03h 36m GMT (Kirkwall). However, Regulus was not visible even with 7x50 binoculars.

By 04h 15m the planet was seen with the naked eye despite the strong twilight.

Circumstances will change significantly over the next four days.

On the 10th (conjunction with Regulus) Mercury rises only about two minutes earlier but the twilight will be less intense and Mercury will have doubled in brightness. Regulus may just be visible to the naked eye. Much will depend on the state of the sky at the time.

Mercury western elongation autumn 2004

Dia:1 Looking east at 03h 30m GMT (UT) 3 September. A track of Mercury is shown for 14 days. Click the image for enlargement

Mercury is at inferior conjunction with the Sun on August 23.

The planet should become observable to the naked eye in the morning sky from the commencement of the second week in September. During the period 4th to 27th Mercury will increase in brightness sixteen fold as its phase goes from 16% to 97 per cent.

No other planet in the solar system displays such rapid changes in brightness and appearance.

As mentioned previously (see archive), Mercury will appear to pass close to the star Regulus on September 10th.

Dia. 1 indicates the path of Mercury relative to Regulus for a week on either side of the conjunction.

Minimum separation (192" S) occurs at 5h 15m on the 10th. Regulus is the faintest of the so-called first magnitude stars.

Remember, Mercury will outshine Regulus some five fold so that it is very likely the star will not be seen without optical aid.

Thus the conjunction will serve as a test for naked eye acuity and will also demonstrate the rapid motion of Mercury (around 10sec in RA per hour) against the star background.

In the telescope on the 10th Mercury will appear like a half Moon. A power of at least x50 will be necessary to see the phase.

Messenger (MErcury Surfaces Space ENvironment GEochemistry Ranging) mission to Mercury.

The probe was launched successfully on August 3 2003, a little later than scheduled.

This is a complex mission since it is planned to use both Venus and Mercury to slow the craft before it is placed into orbit around Mercury. This has been necessary in order to reduce the size of the retrorocket to a minimum.

The "direct route" to Mercury is a great deal shorter than the journey to Saturn and the more remote planets but it is also the planet closest to the Sun. The spacecraft has therefore been designed to minimize disruption from solar radiation to its onboard systems.

There will be, in fact, two flybys of Mercury (and two of Venus) before the probe is finally put into Mercury orbit during early April 2009.

The first and only previous probe to Mercury was Mariner 10. This pioneered the so-called gravity assist trajectory method which has subsequently been used for other missions, notably Voyager and Cassini.

Mariner 10 was launched on November 3 1973.

It achieved its first flyby objectives within a year not without incident. It is to be hoped Messenger will complete its course.

Mercury's visual appearance as a somewhat larger version of our Moon is to some extent misleading.

There are a number of important differences between the two bodies. Messenger will no doubt add to our knowledge of Mercury, but it will almost certainly open new challenges not only in relation to Mercury but to our understanding of the solar system itself.