Advanced Solar Eclipse Planning

WEB

If you haven't already familiarized yourself with the material in the article Solar Eclipse Planning, you should review that first.

What we'll cover

• How to use the eclipse simulator in full page mode
• How to view the lunar limb profile in outline mode
• How to view Baily's beads
• How to adjust the solar radius value
• Simulating the chromosphere

Who is this for?

If you're an experienced eclipse chaser, you likely have already observed many solar eclipse phenomena, such as the diamond ring, corona, and prominences. More experienced eclipse chasers often purposely seek out locations towards the edge of the eclipse path where extended views of Baily's beads and the chromosphere are possible.

That said, Baily's beads are often visible near the central line, but are usually of shorter duration. The key to capturing them is to know what to expect, where to look and when.

The advanced planning tools discussed here show you how to do just that.

If you are already familiar with lunar limb aware eclipse software, you may also want to read the Technical Note.

Accessing the eclipse simulator page

You can expand the mini eclipse simulator shown on the Map page to view it at a much larger size:

When you click the Expand button, you will see a page similar to this - you can take a quick tour of the functionality on the page by clicking 'Start Tour' or the info button:

The eclipse simulator page gives you a larger view of the details of the simulation, plus provides a number of additional tools relevant to advanced eclipse planning.

You may also wish to review these tips on managing graphics performance when using the simulator.

Playback Controls

The eclipse playback controls and key information are displayed at the lower left:

• Play/Pause allows you to play back the eclipse in real time or at an accelerated rate (see Solar Eclipse Planning for more details)
• Skip Back/Forward: skip back or forwards 10 seconds at a time using the double chevron buttons either side of the displayed time - these can be used at any time, including during playback.

Chart and Slider

In the eclipse simulator page, the chart changes as follows:

• The time period spans only C1 to C4
• The chart shows the eclipse magnitude over time
• The time slider resolution is reduced to 0.1 seconds (instead of 10s on the Map page), allowing for much finer grained control - use the left/right cursor keys to change the time by 0.1s per key press when the slider thumb is selected.

Mini map

The mini-map at the lower right shows the location for which the simulation is configured. You can pan and zoom this map just as you can on the Map page itself. In addition you can drag and drop the map pin to change the location without having to return to the Map page.

Use the expand button at the top right of the mini-map to return the Map page.

Simulator Mode

The button at the top right toggles the mode of the simulator between 'photorealistic' (the default) and 'outline'. The screenshot shows the same location as above with outline mode enabled. The selected time is just after C3:

Baily's Beads and Lunar Limb

Let's zoom in on a part of the outline display to see what is shown in more detail:

Baily's Beads are observed primarily (but not solely) around the times of C2 and C3. They are seen when the Sun's photosphere is not fully obscured by the Moon and appears in the valleys of the lunar surface.

In a total eclipse, the duration of Baily's beads is typically shorter because the larger angular size of the Moon fully covers the Sun more rapidly. In an annular eclipse, they can be seen for longer (Francis Baily first documented his beads after observing an annular eclipse in southern Scotland in 1836). If you are near the edge of the central path, you can observe beads for much longer as the limbs of both Sun and Moon remain in close proximity for the duration.

Once the distance from the Sun's limb to the Lunar limb exceeds the depth of the visible lunar valley's we're no longer really looking at 'beads', but are back in the realm of the partial eclipse. However, 'beading' remains possible in the areas (one or two arcs) where the two limbs remain in close proximity. The yellow arc shown above approximately identifies these zones.

Arc of potential beads

The arc of potential beads, shown in yellow, serves two purposes:

• As a simple visual clue to where on the solar limb beads might be observed during an eclipse.
• As a performance optimization for the simulator, such that it only needs to consider a subset of the solar limb in order to display its enhanced bead simulation (described below)

As the time from C2 or C3 increases, the arc will split into two and become shorter, as the cusp angles at the intersection of the solar limb with the lunar limb grow larger. In this image, the area between the arcs is simply exposed photosphere, with no beading possible (i.e. there's no lunar mountain tall enough to break the arc of visible photosphere into two):

Lunar Limb Data

The lunar limb profile data is sourced from the Japanese 'Kaguya' or SELENE mission, and was adapted by David Herald specifically for the purpose of eclipse and lunar occultation calculations. We're grateful to both for making this data available.

The visible limb of the Moon changes over time and by observer location, due to the phenomenon of lunar 'libration' - most easily thought of as 'wobble'. To predict the appearance of Baily's beads during an eclipse, it is necessary first to determine the lunar libration and then to select the correct limb profile data for use in the calculation or simulation. Photo Ephemeris Web does this automatically.

Although the roughness of the lunar limb has a significant impact on the appearance and timing of eclipses, it can be difficult to see without vertical exaggeration. To aid in this, we show the idealised 'smooth' limb of the Moon in blue and an exaggerated relief profile of the actual limb in orange. These are shown at an artificially enlarged lunar radius (×1.2) to avoid obscuring the actual scale limb.

The areas where the limb is rougher or departs farther from the idealised smooth limb are those that give rise to greater numbers and/or longer durations of Baily's beads. This is an important consideration in selecting eclipse viewing locations for the purpose of Baily's beads observation.

Baily's Beads Controls

At the top left of the simulator is a slider to control the notional 'exposure' of the beads. By default it is set to '-16.6EV' (which is the equivalent of off). When enabled, the appearance of the beads changes ("Enhanced Beads"), and some additional controls are available:

Enhanced beads are shown with ±30 seconds of C2/C3 in outline mode and ±12.5 seconds in photorealistic mode. At all other times standard beads (i.e. no 'bead flare') are shown irrespective of the exposure control setting. (We'd love to hear your feedback on this approach - there may be a better way...)

Exposure Slider

The appearance of Baily's beads in photographs and videos is a function of exposure. If you photograph them with a solar filter (which typically is the equivalent of a 16⅔-stop ND filter, with additional UV and IR protection), the beads appear flat and unexaggerated, with no 'flare', as in this photograph of the Oct 14 2023 annular eclipse from New Mexico around the time of C3:

ASE2023 at the Bisti Badlands, New Mexico (© Stephen Trainor)

However, during a total eclipse, and for some observers focused specifically on shooting them during annular eclipses, Baily's Beads are typically photographed without solar filters, in which case they appear more like this:

Location: Madras, Oregon Credit: NASA/Aubrey Gemignani, Source

The exposure slider allows us to simulate both situations.

A Note on numerical accuracy: while the slider is labeled with the values -16.6EV and 0EV, it's best to think of it qualitatively rather than as a strictly accurate exposure control: when switching between shooting with and without a solar filter, other exposure settings in the camera will also change significantly. The exposure control simply attempts to reflect the spectrum of appearance of Baily's beads as commonly seen in photographs and videos.

Partial/Full Limb Switch

When the exposure control is set to -16.6EV, the simulator does no additional special processing to display Baily's beads. It displays the accurate lunar limb (PRO subscription required for future solar eclipses) and whatever pixels of the Sun's disc are not obscured are drawn as normal.

When the exposure control is increased from -16.6EV to any greater value, additional processing is performed to render the 'flared' appearance of the beads. Depending on your computer, the selected resolution (see below) and the circumstances of the eclipse, this can be performance intensive. By default, the simulator will only show 'bead flare' in the zones denoted by the Baily's bead arc(s) (see above).

Typically, this is fine for total eclipses where the area of potential beading is limited, but may give an unexpected visual appearance for annular eclipses, due to the different geometry of such events, in which case you can toggle the control to show the full limb:

In the image on the left, true beading is only possible within the limited area under the yellow arc, but because the Sun's distance from the Moon's limb increases towards the end of the arcs, the flared appearance disappears unexpectedly. If you enable 'full limb' using the button to the right of the exposure slider, the simulator will show 'flare' around the whole limb of the Sun - note that this can be performance intensive (hence the red GPU warning icon).

Resolution Slider

The third control, the beads resolution slider, controls what resolution lunar limb data is used to display enhanced beads. When set to max, all 1800 lunar limb data points (i.e. every 0.2° of axis angle) are used. Unless you are inspecting the data at the very highest zoom levels, this is usually unecessary and a lower setting is recommended. The settings correspond to numerical resolution as follows:

• Max: 1800 points, 0.2°
• H: 900 points, 0.4°
• M: 600 points, 0.6°
• L: 450 points, 0.8°
• Min: 360 points, 1.0°

If you are using an older computer, a setting of Min or L may be necessary to avoid performance issues. When using lower resolution settings, you'll generally obtain better visual results with a higher exposure setting (higher exposure incurs no additional performance penalty, once you're at anything above -16.6EV).

Beads in photorealistic mode

When the simulator is in photorealistic mode, enhanced beads are shown in the same way. At present the simulator does not attempt to show diamond ring or corona (see below for comments on the chromosphere) unless the local circumstances of the eclipse are total. Hence, for a near-but-not-quite-total eclipse, such as this spectacular but short-duration event from 1966, you will see only beads:

The Solar Radius

Much discussion of the solar radius has ensued in recent years. The traditional value of 959.63″ dates from an 1891 paper by Auwers. In recent years, the IAU has changed its official value to 959.22″.

You may wonder whether such tiny differences matter. For the purposes of Baily's beads and lunar limb corrected eclipse contact times, they do. A larger Sun remains eclipsed for a shorter time.

Multiple eclipse researchers have advocated for use of a larger radius value in connection with the observation of solar eclipses. While larger values may not be applicable in other astronomical circumstances (e.g. observations of the Sun made from outside Earth's atmosphere), a larger radius has been found to be consistent with many actual eclipse observations.

In the simulator, we use a value of 959.95″ (Quaglia, Irvine et al.) by default. However, you can adjust this value using the drop down at the top right:

The available values are presented with links to sources.

Unless you're an expert eclipse chaser, it's recommended to stay with the default value of 959.95″.

The Chromosphere

The chromosphere is a relatively thin (~2500km) layer of the solar 'atmosphere' that lies above the photosphere (the intensely bright area that produces our daylight - and also Baily's beads). At times when you can see Baily's beads, and at the start and end of totality, you can often also see the chromosphere. It appears pink due to the frequency of the light produced by hydrogen atoms (H-alpha emissions).

There are no precise measurements of the thickness of the chromosphere - indeed, it may vary at different points on the surface of the Sun and also over time. However, 2,500km is a generally accepted mid-point value. Based on an average 'eclipse' radius of 959.95 arc seconds for the photosphere, we assume the visible chromosphere thickness to be 3.45 arc seconds.

Both simulator modes show the chromosphere at approximately true scale, but with a couple of important differences:

• Photorealism mode: chromosphere is displayed only for total eclipses (magnitude > 1.0), and only from 5s before C2 until 5s after C3; it is displayed irrespective of the Baily's beads exposure setting
• Outline mode: chromosphere is displayed for both total and annular eclipses (not for partial only), from 30s before C2 until 30s after C3 - additionally, it is only shown when Baily's beads exposure is near maximum (optical density between 0.0 and 1.0, or between ~ -3.0 and 0.0 EV)

Here are two examples, photorealistic and outline respectively, both from the 2017 total eclipse (link):

Chromospheric beading

In addition to Baily's beads - broken segments of exposed photosphere - it is also possible for lunar mountains to cause chromospheric beading, for example (photorealistic mode left, outline mode right):

This shows lunar valleys leaving parts of the chromosphere visible during totality at max eclipse in 2017. 

You can use the tool to evaluate different potential shooting locations based on whether they provide longer visibility of the chromosphere, or potentially more interesting visuals with higher numbers of segments of broken chromosphere - a fascinating telephoto eclipse subject!

Note: the screenshots immediately above show one more difference between photorealistic and outline mode. In photorealistic mode, the points of the lunar limb are interpolated to provide a smoother appearance. In outline mode, the raw discrete values are used, so as to avoid any suggestion that the data is higher resolution than it in fact is (0.2° intervals around the lunar limb).

What's next?

• You can look at how we verified the Baily's Beads simulation: Verification of Baily's Beads Simulation
• Check out the Technical Note: Solar Eclipse Functionality
• Consider joining the Solar Eclipse Mailing List for access to discussion by eclipse experts around the world (but bear in mind, it isn't the place for beginner questions - it's more for committed eclipse chasers)