Solar Eclipse, 3 October:

There will be a large partial solar eclipse in Ireland on the morning of 3 October. The magnitude of the eclipse will range from about 62.5% at Fair Head to 74.6% at Mizen Head. This eclipse will be Annular within a track from NW Spain, across extreme NE Portugal, across the rest of Spain passing between Alicante and Valencia, across the Mediterranean into N. Africa, then across NE Africa into the Indian Ocean.

  It occurs in the morning for Irish observers, but not too early, so you can make your apologies in advance and catch it on your way to work.

  It starts at around 08.50 BST for most locations, and reaches maximum at around 10.00. All times below are in BST:

City

Start

P.A. °

Mid

Ends

Mag

Belfast

08.49

285

09.58

11.12

63.8%

Dublin

08.47

287

09.57

11.12

67.1%

L'derry

08.49

284

09.57

11.11

63.9%

Sligo

08.48

286

09.57

11.10

66.4%

Galway

08.46

288

09.56

11.10

69.4%

L'rick

08.45

289

09.57

11.11

70.6%

Cork

08.44

290

09.55

11.11

72.4%

“P.A.” gives the position of the start of the eclipse, measured clockwise from North on the Sun's disc.

  SAFETY: NEVER, EVER, look at the Sun even with the unaided eye, and especially not with ANY sort of optical equipment. A moment's glimpse may be too much, resulting in permanent blindness.   See 'Safety Tips' for more details.

  The Sun will be quite low at the start of the eclipse (only about 9° up in W Ireland), so to see the start make sure you have a good view to the South East. By the time of maximum eclipse, the Sun will be higher in the South East - around 20° for most locations.

  A solar eclipse is easily visible even to the unaided eye, although you MUST use a proper solar filter, such as various specially designed eclipse 'glasses' or hand-held viewers, which are safe if undamaged.

  But some sort of optical aid such as binoculars or a telescope will give a better view. Since a large part of the Sun's disc will be visible, the usual safety precautions for observing the Sun are essential. The danger comes from the brightness of the Sun, not the area that's visible! The following methods are well-proven. Use your widest-field eyepiece first to locate the Sun, either when looking through a filter, or when projecting the image, then switch to a higher power.

  Projection: Project the image of the Sun through a telescope, or half a binocular (cap the other half!), onto a white card held about a foot behind the eyepiece. Shade the card from direct sunlight with a large screen for best contrast. Best suited for small to medium refractors, up to 4” diameter. For larger telescopes reduce the aperture with a mask. This method is good for small group observing, as all can see the image at once.

  Aluminised film: Sometimes referred to as 'Mylar', optical density 100,000:1. The best type is coated on both sides. Don't use it if there are even tiny holes. Baader Astrosolar film has instructions on how to fit it on to the front of the telescope tube. It can also be used on binoculars: one half of the pair is sufficient but tape the cap securely on the other lens for safety. Use only 'visual' grade for viewing!

  Special Projection Solar Scopes: Small, portable specially designed projection systems are available commercially: safe and good for small group observing as all can see the image at once.

  Pinhole viewer: Cut a 2” hole in one end of an old lidless shoebox or similar. Tape aluminium kitchen foil to completely cover this hole. Pierce the foil with a pin to give a tiny round hole. Glue white paper inside the box end opposite the hole. Hold the inverted box up to the Sun so that the light shines through the pinhole onto the paper: you'll see a tiny image of the eclipse.

  H-Alpha filters and telescopes will give a spectacular view of the solar surface, as well as showing the progress of the eclipse.

   What you'll see: Even a large partial eclipse, nor an annular eclipse, cannot compare with a total solar eclipse, because we can't see the Sun's magnificent corona if even a tiny part of the Sun's brilliant disc is unobscured.  But even a partial event is spectacular.

  The Moon will be totally invisible until the moment of First Contact, when a tiny dark 'bite' on the edge of the Sun's disc will appear just a bit to the North of the West edge of the Sun. This bite will gradually enlarge, moving across the SW part of the disc, until the moment of maximum eclipse as seen from each location. It will continue to move across the disc, the amount obscured gradually decreasing, until it leaves the SE edge of the disc, marking the end of the eclipse, at around 11.15 for most locations.

  Careful study will show that the moon's apparent diameter is smaller than the Sun's: hence the annularity.

  If the seeing is fairly steady you'll see the mountains and valleys along the Moon's rim as little irregularities along the edge of its profile.

 The Sun is now near 'solar minimum', so there may not be any sizeable sunspots visible. But it's unusual not to see at least a tiny one, especially if you have a moderate-sized telescope. Also look for the little white patches termed faculae: you'll need good contrast to see these clearly. It's interesting to watch the advancing Moon approach and cover these spots.

  

THE ANNULAR ECLIPSE: A RING OF LIGHT IN IBERIA AND AFRICA

Observers on the centre line of this eclipse will get a different, truly beautiful, view: an annular eclipse. The best options for travelling Europeans are the coast of Spain, or Algeria or Tunisia. Weather prospects in SE Spain are fairly good, and quite good in N Africa.

  Even at maximum eclipse (in N Africa), only 95.76% of the Sun's disc will be covered, so all the normal safety precautions must be taken when observing: the ring of sunlight will still be bright enough to cause serious eye damage.

  The bright annulus will preclude visibility of the beautiful phenomena such as prominences and the corona which are visible during a total eclipse. But there could be brief glimpses of 'Baily's Beads', the small bright beads of sunlight sometimes seen shining through valleys in the Moon's limb at the moments of Second and third Contact. However these won't be nearly as spectacular as during a total eclipse because of the bright ring.

SAFETY TIPS:

Whatever method you use, never look directly at the Sun, even for a moment!

UP FRONT: All solar filter material should be placed over the front of the telescope or binocular, NEVER at the eyepiece end!

TAPE IT ON! Unless the filter screws into the front of the tube, tape the filter-holder onto the tube for safety: it must not come off while observing!

BINOCULARS: use either two filters taped on, or one filter and the other lens capped, both taped on.

CAP THE FINDER! If your telescope has an optical finder, tape a cap (or else a proper solar filter) onto the front of it too: even a finder can blind or burn you!

WELDERS GLASS: The highest density welder's glass (No 14 or higher) is safe for visual observing.

CE MARK: any safe solar filter material will carry this mark, and look for a statement of compliance with EC Directive 89/686/EEC.

1999 ECLIPSE GLASSES/VIEWERS: Relics of the 1999 Total Eclipse will be OK if there are no scratches or pinholes. Cover any small holes with a tiny dab of black paint or a tiny circle of black tape.

DON'T USE: Eyepiece Filters; Photographic Filters (except for photography); polarised filters; smoked glass; exposed colour negative or slide film, or monochrome negative film except totally blackened silver-based emulsion such as Tri-X or HP5; CDs; CD-ROMs; DVDs; inner bags from wineboxes; stacked sunglasses; pinholes (except for projection)  or anything else not specially designed for solar observing.

AVOID PROLONGED STARING at the Sun even though a proper filter; several shorter sessions are preferable.

DON'T GET SKIN OR CLOTHES into the path of unfiltered sunlight coming though a telescope.

DON'T LEAVE the telescope unattended even for a moment if anyone else could get at it.

DON'T LOOK FOR the Sun when in cloud through an unfiltered telescope: it can suddenly re-appear without warning!

ALLOW FOR: children's curiosity, and adults' stupidity!

WARNING SIGNS: If you're having observing visitors, put up some safety warning signs.

IF IN DOUBT: Don't!

HOW TO TAKE IMAGES/VIDEO

CONVENTIONAL PHOTOS: ISO 100 - 200 colour negative film gives quality and wide exposure tolerance.

SLR + TELEPHOTO LENS: Use at least a 200 mm lens to get an acceptable image; 400mm is better; and 1500mm gives an image filling most of a 35mm frame. The Sun's image in mm on a 35mm frame is equal to the effective focal length (EFL) of your lens or telescope, including any teleconverters, Barlows, etc, divided by 109. Thus a 1500mm EFL gives an image 13.76mm across.

FILTERS: The highest ND photographic filters may be OK with a very long EFL and small focal ratio (e.g. F32), particularly when the Sun is low. If you have several filters, try stacking them. Otherwise use special solar photography filters such as Astrosolar Photographic, transmission 1:10,000. Use spot metering if available. With 'average' metering, underexpose by one stop if the solar image is smallish. Ideally experiment on the Sun a week beforehand.

FOCUSING is critical: don't rely on the lens infinity setting! Focus on the Moon's edge. If in doubt take several shots varying focus slightly.

EXPOSURES: Bracket the indicated exposure by a factor of two each way to be safe: film is cheap.

DIGITAL STILL CAMERAS: Even the simplest ones, and webcams, can be held up to the eyepiece of a filtered telescope: auto exposure will do the rest. Check each image and try again if necessary.

DIGITAL ZOOM will enlarge the image but degrade quality; optical zoom is OK.

ADAPTORS: various adaptors to connect both SLR and digital cameras to the eyepiece mount are available, and will give better results.

PHOTOGRAPHING A PROJECTED SOLAR IMAGE gives reasonable results, but contrast may be low.

VIDEO: Some top digital video cameras have huge zooms, giving a large enough image. Otherwise shoot through a filtered telescope.

MOUNTING: With long EFLs, the slightest shake is ruinous. Use the steadiest mount possible. Use a cable release, and mirror-lockup if available, to avoid shake.

  [This article is based on one I was asked to write for the October issue of the BBC Sky At Night Magazine]

Terry Moseley