August 12, 2026 • Sunset Eclipse over Sierra de Gredos

Solar Eclipse Astrophotography & Astrophysics

On the evening of August 12, 2026, the sun dropped toward the western horizon in a deep 99.34% partial eclipse. By shooting linear 14-bit RAWs through a homemade solar filter with zero sensor clipping, this photographic sequence became a calibrated instrument to measure solar limb darkening and atmospheric extinction.

Default Observation: Sierra de Gredos, Ávila (40.35°N, 5.25°W, 1500m) • Configurable

Measured Maximum

99.34%

+0.42% delta Boundary Calibrated

Sidereal Drift Rate

3.635 px/s

53.22° Celestial Descent Vector

Ephemeris Time (CEST)

20:32:38

±7s Besselian Clock Reduction

Airmass Traversed

2.92 → 8.59

Rayleigh & Aerosol Reddening

Boundary Offset δ

0.818 px

Calibrated on Uneclipsed Frames

Location-Dependent Sky & Horizon Ephemeris

Solar altitude, crescent tilt, and obscuration change dramatically depending on your exact geographic position. Explore how the eclipse appeared across Spain and Europe, or enter your own coordinates.

19:30 CEST (C1 Onset) 20:32:38 (Maximum) 20:50 CEST (Sunset)

Select Observation Preset

Shoot location • Deep partial eclipse (99.34%) setting behind the western mountain crests of Gredos.

Custom Coordinates

Latitude (°N)

Longitude (°E/W)

Elevation (m)

Local Ephemeris Telemetry

Local Maximum Time20:32:38 CEST

Local Maximum Obscuration99.34%

Apparent Solar Altitude8.26°

Parallactic Angle (Crescent Tilt)48.2°

Local Optical Airmass X(z)6.85

Why does the crescent tilt? As the sun sets, the celestial equator cuts into the local horizon at an angle equal to (90° - Latitude). In Gredos (40.35°N), the solar axis is tilted ~48° relative to the horizon, rotating the lunar bite from a high-afternoon 'horn' to a horizontal sunset 'smile'.

Key Narrative Milestones (Click to Jump)

Interactive Frame Inspection Studio

Step through the 58 centrable eclipse frames and 7 sunset coda frames. Every frame is registered with sub-pixel circle fitting and tone-mapped in linear light.

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Color Grade

Exposure Policy

Calibrated Frame Telemetry

RAW Frame StemDSC03678

Calibrated Obscuration99.34%

True Local Time20:32:38 CEST

Solar Altitude8.26°

Camera Settings1/1600s • ISO 100

ClassificationECLIPSE (Centred)

2D Lunar Transit Vector Recovery

A 1D light curve cannot reveal which direction the moon crossed the sun. However, on any registered solar disc, the area-weighted first moment (centroid) of the lit crescent points strictly opposite the moon.

The Line of Centres Theorem

Because the lit crescent is the solar disc minus the circular overlap, its area centroid lies exactly on the line connecting the Sun and Moon centres:

$$\vec{c}_{\text{lit}} = -\frac{A_{\text{overlap}}}{A_{\text{lit}}} \vec{c}_{\text{overlap}}$$

Lunar Velocity Vector3.635 px/s (22.5° Transit Track)

Simulated Obscuration99.34%

Separation Distance0.051 R_sun

By tracking this centroid across 58 frames, we reconstruct the moon's 2D trajectory vector with R² > 0.999 linearity, independently confirming the 0.051 R_sun closest approach distance.

Solar Astrophysics from Camera RAWs

Because the sensor never saturated (brightest photosphere sat 2.8 stops below clipping), the raw pixel counts are strictly proportional to photon flux, allowing real solar physics measurements.

Photospheric Radial Temperature Model (6400 K → 4500 K)

Photospheric Temperature Gradient & Limb Darkening

When looking at the centre of the sun, our line of sight reaches deeper into hotter layers (6400 K). Toward the limb, we only see higher, cooler layers (4500 K). Because Planck's law causes hot gas to emit much more blue light, the solar limb darkens much faster in blue wavelengths than in red.

$$\frac{I(\mu)}{I(1)} = 1 - u_1(1-\mu) - u_2(1-\mu)^2$$

Blue Channel (B)u1 = 0.652, u2 = 0.108

Green Channel (G)u1 = 0.570, u2 = 0.124

Red Channel (R)u1 = 0.507, u2 = 0.134

Edge Contrast I(0)/I(1)Blue: 0.239 • Red: 0.360

Computational Astrophotography & RAW Pipeline

How consumer photographic hardware was transformed into a high-precision scientific instrument through sub-pixel geometry, sidereal drift modeling, and linear photometric calibration.

Step 01 • RAW Unprocessed Sensor

14-Bit Linear Sensor Extraction

Direct conversion of Sony ILCE-6400 RGGB Bayer CFA data (B0 = 512, saturation at 16383). No camera tone curves, sharpening, or sRGB gamma applied, preserving strict photon flux linearity.

Step 02 • Limb Detection

Sub-Pixel Geometric Inversion

Algebraic Kåsa-Taubin circle fitting on the uneclipsed solar limb. Determines the exact optical center (x0, y0) and solar radius (R = 238.43 px) with residual RMS < 0.04 px.

Step 03 • Drift Decomposition

Sidereal Drift Decomposition

28 manual tripod re-aims were decomposed into a continuous piecewise-linear celestial drift model at 3.635 px/s, recovering absolute temporal alignment (R² > 0.9998).

Step 04 • Boundary Calibration

Boundary Offset Paradox (δ)

A sub-pixel threshold offset of δ = 0.818 px was calibrated on uneclipsed frames, correcting a +0.42% perimeter-to-area systematic bias on thin crescent measurements.

Step 05 • Tone Grading & LUTs

Planckian Tone Grading & LUTs

Möbius tone-shoulder compression and 3 calibrated grading LUTs: Warm Gold (solar atmosphere), True Solar Neutral (5778 K blackbody), and Monochrome (pure photon density).

Dynamic Sequence Poster Architect

Create astronomical sequence posters where distance along the axis is strictly proportional to physical time (x ∝ t). Renders real linear RAW image crops across all layout modes.

Layout Geometry

Number of Solar Discs: 16

Blending Mode

Color Grade

The Camera, Homemade Filter & Sidereal Math

How consumer photographic gear was turned into a high-precision astronomical sensor.

Camera Body

Sony α6400 (APS-C 24.2 MP, uncompressed 14-bit linear RAW)

Telephoto Lens

Sony E 55–210mm F4.5–6.3 OSS at 210mm (315mm equiv.)

Solar Filter

Homemade Baader AstroSolar Safety Film filter (ND 5.0, 1/100,000 transmission)

Mounting & Drift

Stationary manual tripod. 28 manual pointing re-aims were decomposed into a 3.635 px/s sidereal drift model, recovering sub-pixel alignment for all frames.

Mathematical Foundations & Ephemeris Derivations

Formulations derived from first principles for clock synchronization, celestial mechanics, and radiative transfer.

Besselian Polynomial Reduction

Local contact times $T$ are computed from the Besselian fundamental plane $(x, y, d, \mu, l_1, l_2)$:

$$(x - \xi)^2 + (y - \eta)^2 = (l_1 - \zeta \tan f_1)^2$$

Piecewise-Linear Sidereal Drift Decomposition

Within each continuous tracking block $k$, the solar center in sensor pixels $(x_s, y_s)$ evolves as:

$$\vec{x}_s(t) = \vec{x}_0^{(k)} + \vec{v}_{\text{drift}} \cdot (t - t_0^{(k)})$$

Möbius Tone Shoulder Compression

Linear sensor DNs are mapped to display luminances without clipping highlights:

$$f(x) = \frac{x(1 + ax)}{1 + bx}$$

IGN / OAN Spain Calibration & Ephemerides

Topocentric solar/lunar coordinates, contact times, and duration tables across Spain are calibrated against official Besselian elements published by the Instituto Geográfico Nacional (eclipses.ign.es).

$$\tan P = \frac{-\xi'}{\eta'}, \quad \sin d = \frac{\zeta - \mu}{\sqrt{x^2 + y^2}}$$