The evening sky in spring

Spring is always the best time from the northern hemisphere to observe any planets visible in the evening sky. During early to mid April 2002 there was a rare, outstanding display of all the naked eye planets in the evening. This year another opportunity will arise enabling all the naked eye planets to be seen in the sky at one and the same time, though not so spectacularly as in 2002.

Dia 1: Sky looking west at 19h 00m GMT on March 22. Click the image for an enlargement

From about March 10 it should be possible to see the planet Mercury low down in the west shortly after sunset. This day it will be a little brighter than Sirius but over the succeeding days, as Mrecury climbs higher into the sky, so it will fade to be only half as bright at March 22. By this date the crescent Moon will have joined the retinue of planets - see Dia. 1.

It has not been possible to include Jupiter in the diagram but in the real sky one has only to look further east (left) in order to find the brilliant planet. In order of brightness we have:

• Venus (-4.3);
• Jupiter (-2.4);
• Mercury (-0.9);
• Saturn (0.0)
• Mars (1.3).

Apparent magnitudes are given in brackets - for an explanation of magnitudes see archives.

Vernal Equinox

2004 being a leap year, the Spring or Vernal Equinox occurs early on March 20 at 06h 49m. This day the Sun passes into the northern celestial hemisphere and will remain above the horizon for 12 hours. From the Northern Isles twilight will become progressively dominant such that by April 24, through to August 18, astronomical twilight lasts all night. See archives feature on twilight.

Occultation of Mars - March 26

There will be an occultation of Mars by the Moon (phase 23%) commencing 00h 15m 45s on the morning of March 26. From Orkney the planet will have an altitude of just 3° 16' in azimuth 310° at this time. Those with a clear NW horizon should take the opportunity to observer the event. The disappearance takes place at the dark limb of the Moon and should be readily visible. Binoculars or a small telescope will add greatly to the ease of observation. The circumstance of the occultation improve with increasing latitude. For Lerwick the circumstances are marginally better (offset by longitude) where the Moon sets later than at Kirkwall by around 3 minutes. (Moonset is at 00h 59m 34s at Kirkwall.)

JV
04/03/04

Jupiter at opposition, 2004

This year Jupiter will be at opposition on March 4th.

Dia 1: Looking south at midnight, 2004 March 4. Click the image for an enlargement

The chart (dia. 1) shows the sky looking south at a little after midnight on the 4th. The Moon will be seen a little to the west (right) in the constellation Cancer.

At magnitude -2.4, Jupiter is approximately 75% as bright as at his nearest on a favourable opposition (October 1999, for example). Even so, the planet makes for a splendid sight in the night sky right through until early dawn twilight begins to take over (see fig. 1).

Jupiter rises at 17h 27m on the 4th, and at 16h 54m a week later. This is slightly greater than the sidereal rate for a star and is due to the fact that Jupiter is in retrograde motion with an accompanying slight increase in declination. The observer will have the impression that the planet is heading back towards Regulus. By May 6th, having travelled some 5º, it will reach a stationary point: thereafter it will resume its forward motion towards Virgo.

Fig. 1: The sky looking west on February 19 at 0545. The constellation Leo occupies most of the picture to the centre right with Jupiter below (centre). Regulus at the base of the "sickle" is the next brightest object low down to the right. Part of the sprawling Virgo appears on the left. A 20 second exposure ISO 800, f/2.5 and digitally processed. Photo: John Vetterlein.

This passage into Virgo signals the end of a period of some six years during which Jupiter has dominated our autumn and winter skies. We shall have to wait until the year 2011 before Jupiter again crosses the celestial equator into the northern celestial hemisphere. Therefore, make the most of the time left this winter and spring to observe Jupiter.

Observing Jupiter

It is quite easy to follow the movement of the four Galilean satellites using a small telescope or binoculars. They may be seen with a power of x4, such as is afforded by an opera glass. A pair of 8x30 binoculars, if mounted on a tripod or other suitable support, will enable an observer to watch the change in position of the satellites relative to one another during the course of a few hours or from night to night. A power of around x20 will show the two atmospheric bands running parallel to the planet's equator (see fig. 2).

Fig. 2: Jupiter 2004 February 19, 03h 15m 175mm Maksutov telescope, 1/90 sec. exposure, ISO 400, digital imaging and processing. Photo: John Vertterlein.

An 80mm instrument at x80 is sufficient to show the great red spot. For shadow transits of the satellites themselves a 100mm instrument should suffice.

However, it is not possible to be categorical about such requirements since so much depends on the quality of the sky at the time. Good seeing is essential when using high powers with any instrument.

And a rule of thumb for all observing is to ensure the instrument is adequately supported to resist vibration from wind and touch. Most amateur astronomical equipment today tends to be manufactured with portability in mind. In the islands we seldom experience calm conditions; a shelter or improvised observatory is more or less essential if good results are to be achieved using high powers.

[See archives for last year's feature on Jupiter.]

Mars

Mars is currently crossing the local meridian shortly before sunset. At magnitude 1 it is comparable in naked eye appearance to the bright star Aldeberan, about 25º east and at almost the same declination. (Mars is in forward motion and will pass well north of the star during the first week of April).

This much improved altitude for Mars offers a good opportunity for those with suitable equipment to observe the planet in daylight or strong twilight, at least for the next month. Recent observations here on Rousay bear out my findings over four decades that seeing can be excellent during daylight and early evening at this time of year in the northern hemisphere.

On February 20, with the planet high in the sky, it was possible to employ powers of x400 on Mars using the 175mm Maksutov an hour before sunset. Despite its small apparent diameter (6 arc seconds and diminishing) it was quite possible to make out a suggestion of some surface detail.

JV 21/02/04