Eclipses: the geometry and science of solar and lunar eclipses, and the view from Türkiye

At a glance

An eclipse happens when three bodies line up almost exactly. When the Moon passes between the Sun and Earth we see a solar eclipse; when Earth passes between the Sun and the Moon and casts its shadow on the Moon we see a lunar eclipse. A solar eclipse can only occur at new Moon and a lunar eclipse only at full Moon.

Sources and dates: Moon and Sun data from the NASA NSSDCA fact sheets (Moon: last updated 11 January 2024; Sun: 9 May 2024). The draconic month and eclipse seasons are from F. Espenak’s EclipseWise page; the saros period from the NASA GSFC eclipse website.

Sources: [1] [2] [3] [5]

The geometry of shadow: umbra and penumbra

The Sun is not a point but a broad disc of light, so two kinds of shadow form behind the Moon (or Earth). The umbra is the narrowing cone in which the whole solar disc is hidden. Around it lies the widening penumbra, where only part of the Sun is covered. Beyond the tip of the umbral cone is the antumbra; an observer there sees the Moon as a dark disc inside the Sun, leaving a bright ring around it.

During a solar eclipse, the narrow strip where the umbra touches Earth’s surface is the “path of totality”, typically a few hundred kilometres wide. The penumbra covers an area thousands of kilometres across, and everyone inside it sees a partial eclipse. In a lunar eclipse Earth’s shadow is far larger than the Moon, so everyone who has the Moon above the horizon sees it at the same moment.

Sources: [2] [4]

Why not every month? Nodes and eclipse seasons

The Moon passes through new and full phases every 29.5 days, yet eclipses happen only a few times a year. The reason is that the Moon’s orbit is tilted by about 5.1° to the plane of Earth’s orbit around the Sun (the ecliptic). At most new Moons the Moon passes slightly “above” or “below” the Sun and its shadow misses Earth. The two points where the orbital planes intersect are called nodes; an eclipse needs a new or full Moon close to a node.

According to Espenak’s calculations, if new Moon falls within about 17° of a node (the limit ranges from 15.39° to 18.59° depending on orbital eccentricity), a solar eclipse is visible from somewhere on Earth. This window, which opens as the Sun passes a node and lasts about 34.5 days, is an eclipse season. Each season brings one or two solar eclipses, and the midpoints of successive seasons are about 173.3 days apart. As a result, every year has at least two and at most five solar eclipses. Because the line of nodes slowly regresses, eclipse seasons arrive a little earlier in the calendar each year.

Sources: [1] [3] [5]

Types of eclipse

Solar eclipses

• Total: the Moon covers the whole solar disc; the sky darkens and the corona, the Sun’s outer atmosphere, becomes visible. It is seen only from the strip crossed by the umbra.
• Annular: the Moon is near apogee and looks smaller than the Sun, leaving a bright ring around the disc. The observer is inside the antumbra.
• Hybrid: because Earth’s surface is curved, the eclipse is total along some parts of the shadow path and annular along others.
• Partial: only the penumbra touches Earth and the Sun appears as a crescent. Wide regions outside the paths of total and annular eclipses also see a partial phase.

Lunar eclipses

• Penumbral: the Moon passes only through Earth’s faint penumbra; the dimming is often hard to notice by eye.
• Partial lunar eclipse: part of the Moon enters the umbra; a dark “bite” is easy to see.
• Total lunar eclipse: the whole Moon is inside the umbra. It does not disappear: sunlight refracted through Earth’s atmosphere, with its blue component scattered away, lights the shadow, and the Moon turns copper or red.

Sources: [4] [13]

Lab: total or annular?

In the calculator below, change the distances of the Moon and the Sun and compare the apparent sizes of the two discs. Choose the “Moon at apogee, Earth at aphelion” preset and you will see that the Moon cannot cover the Sun, so the eclipse would be annular; the note under the lab explains the assumptions behind the calculation.

If the Moon’s apparent diameter exceeds the Sun’s, a central eclipse is total; if smaller, annular. That the two are so close is a coincidence, and not a permanent one: tides are very slowly moving the Moon away from Earth.

Sources: [1] [2]

The saros cycle: the rhythm of eclipses

The saros, known since antiquity, equals 223 synodic months (6,585.3223 days). This matches 242 draconic months (6,585.3575 days) and 239 anomalistic months (6,585.5375 days) to within a few hours. After one saros the Moon is back at the same node and at almost the same distance from Earth, so the eclipse repeats with nearly the same geometry.

The leftover of about 8 hours, however, means that Earth turns an extra third of a rotation and the new eclipse path shifts about 120° to the west. A saros series lasts 1,226 to 1,550 years and contains 69 to 87 eclipses. A neat example for Türkiye: the total eclipse of 29 March 2006, which ran from Antalya to the Black Sea, belonged to Saros 139; one saros later, the next member of that series crossed North America on 8 April 2024. The successor of the 11 August 1999 eclipse (Saros 145) crossed the entire United States on 21 August 2017.

Sources: [5] [17]

The science of totality: from Baily’s beads to Einstein

Just before and just after totality, sunlight streams through valleys along the Moon’s rugged edge; this string of bright points is known as Baily’s beads. The last or first glint, set against the glowing inner corona, gives the impression of a “diamond ring“. Then, for a few minutes, the corona, the chromosphere and prominences at the limb, normally hidden by the Sun’s bright surface, become visible.

These brief moments have hosted major discoveries. At the 1868 eclipse Jules Janssen, observing with a spectroscope in India, saw a yellow line in light from the Sun’s edge slightly below 588 nm, close to but distinct from sodium’s. Norman Lockyer independently detected the same line that year and attributed it to a new element. Named after the Sun, helium remains the only element first observed on another celestial body.

The eclipse of 29 May 1919 was the first great test of general relativity. Einstein’s theory predicted that starlight grazing the Sun’s limb should be bent by 1.75″; a Newtonian calculation gives half that. Two expeditions organised by Arthur Eddington and Frank Dyson photographed stars near the Sun during the eclipse, on the island of Príncipe and at Sobral in Brazil. The Sobral data gave a deflection of about 1.98″ and the Príncipe data about 1.61″, both consistent with Einstein’s value. Although the choice of plates was later debated, both measurements lie within two standard deviations of Einstein’s value and clearly away from the Newtonian one.

The corona’s temperature of millions of degrees and the origin of the solar wind remain open questions. Because atmospheric scattering swamps the inner corona in ground-based coronagraphs, natural eclipses remain one of the best ways to see the corona close to the solar surface.

Sources: [6] [7] [8] [9] [10]

Safe viewing

It is never safe to look at the Sun without specialised protection. The only exception is the brief total phase of a total eclipse, when the Sun’s bright face is completely covered; protection must go back on as soon as any sliver of the Sun reappears. Partial and annular phases must be watched through eclipse glasses or handheld solar viewers that meet the international ISO 12312-2 standard. Ordinary sunglasses, however dark, are not safe.

• Inspect viewers before use; discard any that are scratched, torn or damaged, and supervise children.
• Never look through binoculars, a telescope or a camera while wearing eclipse glasses: the light concentrated by the optics can burn through the filter and reach the eye. These instruments need dedicated solar filters fitted to the front.
• Without glasses, pinhole projection is a safe option: with the Sun behind you, cast the image through the hole onto a surface, and never look at the Sun through the hole.

Sources: [11]

Eclipses visible from Türkiye (2026–2030)

The table below summarises events relevant to Türkiye from NASA’s 2021–2030 eclipse lists. Turkish time (TRT) is UTC+3. For lunar eclipses, visibility has been assessed by whether the Moon is above the Turkish horizon at the relevant times.

Dates, types and magnitudes are from the NASA GSFC (F. Espenak) decade lists; lunar eclipse contact times from EclipseWise pages; 2030 path data from the EclipseWise path table. City obscuration values for 2 August 2027 are approximate figures computed by SpaceTRC from NASA’s published Besselian elements (İzmir and Antalya ≈ 71%, Istanbul ≈ 59%, Ankara ≈ 57%, Van and Trabzon ≈ 42%) and may be off by a few points. The Moon’s altitude was assessed with an approximate calculation.

The eclipse of 2 August 2027 is one of the longest total eclipses of the century: greatest eclipse occurs at 10:06 UT at 25°30′N, 33°11′E, near Luxor in Egypt, where totality lasts 6 minutes 22 seconds and the path is about 258 km wide.

Sources: [12] [13] [14] [15] [16] [19] [20]

Historic eclipses in Anatolia

The eclipse of Thales (585 BCE, tradition). According to the first book of Herodotus’ Histories, during a war between the Lydians and the Medes day suddenly turned to night; both sides stopped fighting and made peace. Herodotus writes that Thales of Miletus had foretold this “loss of daylight” to the Ionians. NASA’s list of historical eclipses matches the event with the total eclipse of 28 May 585 BCE. However, how Thales could have made such a prediction, and the details of the story, are debated among historians; the event is best treated as a strong tradition rather than a firm scientific prediction.

11 August 1999. The last total solar eclipse of the second millennium cut diagonally across Türkiye. The umbra reached the Black Sea coast at 11:21 UT, crossed the interior and arrived at the south-eastern border at 11:45 UT. Turhal experienced 2 minutes 15 seconds of totality; Ankara, just outside the path, saw a partial eclipse of magnitude 0.969.

29 March 2006. After crossing Africa, the umbra entered Asia over Türkiye’s Mediterranean coast. The path, about 171 km wide, passed over Antalya, Konya, Aksaray, Nevşehir, Kayseri, Sivas, Tokat, Amasya, Ordu and Giresun. Totality lasted 3 minutes 48 seconds on the central line and reached 3 minutes 36 seconds at Konya.

Sources: [8] [18] [17]

Current research (2024–2026)

8 April 2024: a total eclipse crossed Mexico, the United States and Canada; the umbra made landfall on Mexico’s Pacific coast and left from Newfoundland. NASA invited the public to join citizen-science projects during the eclipse.

Artificial eclipses with Proba-3 (2024–2025): ESA’s Proba-3, launched on 5 December 2024, consists of two satellites flying about 150 metres apart; one carries a disc that blocks the Sun, the other images the corona. Holding their relative position to the millimetre, the pair can sustain the eclipse for up to six hours of each 19.6-hour orbit. The first images were released on 16 June 2025. Compared with the few minutes of a natural eclipse, these hours-long “eclipses on demand” allow regular monitoring of the inner corona.

12 August 2026: this year’s total eclipse was watched from Greenland, Iceland and Spain. The next major opportunity is the total eclipse of 2 August 2027, with more than six minutes of totality.

Sources: [10] [12] [14] [21]

Data for researchers

• NASA GSFC eclipse website: decade lists, saros tables and Besselian elements for each eclipse, based on F. Espenak’s Five Millennium Canon of Solar Eclipses (2021–2030 list).
• EclipseWise: an updated version of the same calculations, with path tables, interactive maps and local-circumstance calculators (eclipsewise.com).
• Your own calculation with Besselian elements: local contact times and obscuration can be computed from the published polynomial coefficients (x, y, d, l₁, l₂, μ) with a few lines of code; the 2027 city values on this page were obtained this way.

Sources: [12] [14] [15]

Known vs open questions

Established

• Eclipse times and paths can be computed centuries ahead; the main uncertainty is Earth’s irregular rotation (ΔT).
• Eclipses happen only a few times a year because of the 5.1° tilt of the Moon’s orbit.
• The 1919 measurements and far more precise later experiments confirm that gravity bends light by the amount general relativity predicts.

Open questions

• Why is the corona millions of degrees hotter than the surface, and where exactly is the solar wind accelerated?
• Which ancient eclipse records (such as the Thales story) can be confidently matched to a specific event?
• To what extent can artificial eclipses in space replace the inner-corona views that natural eclipses provide?

Related on this site

• Research dossier: The Sun and space weather
• Research dossier: History of astronomy
• The Sun
• The Moon
• Astronomy fundamentals
• Telescopes and observing
• Space guide: Orbital mechanics
• Orbit lab