Eclipses
Solar eclipses: how they happen, where to see them and safe viewing
At a glance
A solar eclipse occurs when a new moon aligns closely enough with the Sun to cast its shadow on Earth. A narrow region can experience totality or annularity, while a much larger area sees a partial eclipse. Apphemeride calculates local contacts, maximum and duration so you can understand the circumstances at your location.
Why an eclipse does not happen every new moon
A new moon occurs every lunation, but the lunar orbit is inclined by about five degrees to Earth’s orbital plane. Most new moons pass north or south of the exact alignment needed to cover the Sun.
An eclipse requires the Moon to be near a node, one of the two places where its orbit crosses the ecliptic. Suitable alignments occur in eclipse seasons, approximately twice a year. Across the world there are at least two and at most five solar eclipses in a year.
That global frequency does not describe one town’s opportunities. Each eclipse affects only a region, and the central path is far narrower than the partial-visibility area.
Similar geometries recur after a **saros**, about eighteen years, eleven days and eight hours. The extra hours shift the next geographical view approximately a third of a turn westwards. A repeated saros does not mean an eclipse will return to the same city.
Total, annular and partial
The shadow contains distinct regions. In the **umbra, the lunar disc blocks the whole solar photosphere. In the penumbra, only part is hidden. Beyond the umbra’s tip lies the antumbra**, where the foreground Moon appears smaller than the Sun.
- A total solar eclipse occurs where the umbra reaches the surface. Totality lasts seconds or minutes and reveals the faint corona.
- An annular eclipse leaves a bright ring because the apparent lunar disc is too small to cover the Sun completely.
- A partial eclipse is an incomplete overlap from the observing location. It may be part of a globally partial event or the view outside the central path of a total or annular one.
The parameter gamma describes how far the shadow axis passes from Earth’s centre in Earth-radius units. Near zero the passage is central; values near the edge correspond to more grazing geometry. Gamma alone does not say what will happen at a New York address or a Grand Canyon viewpoint.
Magnitude is not obscuration
Solar-eclipse magnitude concerns the apparent diameters and penetration of the discs. Obscuration is the fraction of the Sun’s area hidden. They are different quantities and should not be presented as interchangeable percentages.
Overlapping circular areas do not change linearly with overlap along a diameter. A quoted magnitude therefore cannot simply be relabelled as the percentage of sunlight or surface blocked.
The remaining photosphere and conditions in the sky determine the visual impression. Even very high obscuration leaves dangerous direct sunlight. A dim landscape is not proof that totality has begun, and should never be used to decide whether protection can be removed.
Local circumstances matter more than a country label
The shadow sweeps across Earth and draws a path. Two places in the same country may see different types, durations or phases, while a place outside the path may see only a partial eclipse or nothing.
For planning in the United States, do not treat “visible in the US” as a local prediction. A path through one region does not cover New York and Arizona alike. International travellers need the same care with the chosen site and time zone.
Check whether the location is central, the Sun’s altitude, and whether sunrise or sunset interrupts the event. A canyon wall, hill or building can hide a phase that occurs above an ideal mathematical horizon.
Indirect viewing is another option. A small hole projects an image of the Sun onto a screen; leaf gaps can act as many small projectors. Look at the projected image, never through the hole towards the Sun. Optical projection requires an instrument and setup suitable for solar use.
How Apphemeride answers this
How Apphemeride answers this
Apphemeride presents global beginning, maximum, end, gamma, magnitude and eclipse type. Locally it provides the applicable C1–C4 contacts, maximum and duration, including circumstances such as an eclipse already in progress at sunrise.
The calculation uses Besselian elements and ΔT. Eclipse data in the Sun and Moon cards are free. The full version adds local simulation, eclipse paths on Earth with a central line and time marks, equal-magnitude lines and upcoming events with reminders.
Use the simulation to study the sequence before observing. Complete the astronomical plan with the actual horizon, access arrangements and weather conditions. Explore solar and lunar eclipses in Apphemeride.
Frequently asked questions
How often does a solar eclipse occur?
There are between two and five worldwide each year. Totality at one fixed place is much rarer because the central path is narrow and changes between events. A global count is not a local observing calendar.
How do I find an eclipse visible from my location?
Select the eclipse and actual observing position in the app. Check path membership, local contacts, altitude and horizon. A national or regional headline is not enough to establish the phases visible at your site.
What protection do I need?
For direct viewing, undamaged eclipse glasses complying with ISO 12312-2 from a reliable source. Magnifying instruments need suitable front-mounted solar filters; glasses do not replace them. Protection remains in place during all partial and annular phases.
Why not every new moon?
The inclined orbit usually carries the Moon above or below the required alignment. Only a new moon close enough to a node can cast its shadow on Earth.
