How Total Solar Eclipses Work: Magnitude, Saros, and Why Totality Is So Rare
The 2028 eclipse is Sydney's first totality since 1857. Understanding why the wait was 171 years — and why the show is so brief — makes standing in the shadow even better.
Quick answer: A total solar eclipse happens when the new Moon passes exactly between Earth and the Sun while close enough to appear larger than the solar disc, casting a shadow track rarely more than a couple of hundred kilometres wide. Because that narrow track lands somewhere different each time, any single city waits centuries on average between totalities - which is why Sydney's 22 July 2028 eclipse is its first since 1857.
Why do total solar eclipses happen at all?
Total eclipses exist because of a cosmic coincidence with no known parallel: the Sun is about 400 times wider than the Moon, and also about 400 times further away. The two discs therefore appear almost exactly the same size in our sky. When the Moon passes precisely between Earth and the Sun, it can cover the bright solar surface completely — revealing the corona, the Sun's ghostly outer atmosphere, which is normally drowned by a million times more glare.
The Moon's shadow has two parts. The wide outer penumbra creates the partial eclipse seen across all of Australia and Aotearoa New Zealand in 2028. The narrow inner umbra — the zone of totality — is a spot rarely more than a couple of hundred kilometres across that races along the ground at more than a kilometre per second. Its 2028 track, from the Indian Ocean across Australia to beyond the South Island, is mapped on the interactive path viewer.
Why isn't there an eclipse every month?
The Moon is new every 29.5 days, but its orbit is tilted about five degrees to Earth's orbit around the Sun. Most months, the new Moon's shadow sails harmlessly above or below Earth. Only when a new Moon occurs while the Moon is crossing the plane of Earth's orbit — at points called nodes — does the shadow strike the planet. Those alignments cluster into two eclipse seasons a year, about six months apart, which is why eclipses come in loose pairs of solar and lunar events.
Even then, most solar eclipses are partial or annular. Totality additionally requires the Moon to be near the closer part of its slightly oval orbit, so its disc is genuinely big enough to cover the Sun. Miss that condition and you get an annular "ring of fire" eclipse instead — beautiful, but never safe to view unfiltered and missing the corona entirely.
What do magnitude and obscuration actually mean?
Two numbers describe how deep an eclipse is, and they are routinely confused:
- Magnitude is the fraction of the Sun's diameter covered by the Moon. Magnitude 1.0 or greater means totality is possible; Sydney's 2028 magnitude of 1.025 means the Moon's disc overspills the Sun's, with margin that pushes totality to 3 minutes 48 seconds.
- Obscuration is the fraction of the Sun's area hidden — always lower than you would guess from the magnitude. Melbourne's 2028 magnitude of 0.844 leaves a fat crescent of Sun uncovered, daylight barely dimmed, and glasses mandatory throughout.
This is why the difference between 99 percent and 100 percent is not one percent. Until the last of the photosphere vanishes, the sky stays bright and the corona stays invisible. Totality is a switch, not a dial — compare every city's magnitude on the city guides hub.
The saros: how eclipses repeat
Eclipses are not random. After one saros — 6,585.3 days, or about 18 years and 11 days — the Sun, Moon, and node return to nearly identical geometry, and a near-copy of the eclipse recurs. Each family of eclipses sharing this rhythm is a numbered saros series; the 22 July 2028 eclipse belongs to saros 146.
The tell-tale extra third of a day in the cycle is why successive eclipses in a series don't repeat over the same ground: Earth rotates another 120 degrees before the shadow returns, stepping each eclipse a third of the way around the planet. Only after three saros periods — about 54 years — does a series come back to roughly the same longitudes, and even then shifted in latitude. A saros series lives for twelve to fifteen centuries, beginning with glancing partial eclipses near one pole, maturing into long totalities, and fading out near the other pole.
Why did Sydney wait 171 years for totality?
Average the numbers globally and a total eclipse touches any given point on Earth roughly once every 375 years. The umbra is a thread a couple of hundred kilometres wide draped somewhere different on each pass, and the oceans soak up most of them. Sydney's last totality was in 1857; the 2028 event ends a 171-year drought — slightly better than the global average, which is exactly the kind of luck worth taking.
Australia as a whole has fared better than its cities. The famous 1922 Wallal eclipse in Western Australia produced the starlight-deflection measurements that helped cement Einstein's general relativity. Cairns in November 2012 gave Queensland the country's most recent totality. And the near future is remarkable: after 2028, another total eclipse crosses South Australia and New South Wales on 25 November 2030, with further Australian totalities in the late 2030s. Eclipse chasers already call it the Australian decade.
What will you actually see, stage by stage?
Knowing the sequence turns a spectacle into a story. First contact: the Moon takes its first notch out of the Sun — glasses on, and for the next 80 minutes the bite deepens. In the final ten minutes, light turns metallic, shadows sharpen strangely, and the temperature slides. The last minute: the western sky darkens as the umbra approaches; on flat ground you can watch the shadow arrive. Second contact: the last bead of photosphere flares as the diamond ring, breaks into Baily's beads shining through lunar valleys — then totality. Glasses off. The corona spans several solar widths, pink prominences fringe the black Moon, planets appear, and a sunset glows around the entire horizon. At third contact the diamond ring flashes on the opposite edge, glasses go back on, and the whole play runs in reverse to fourth contact. The first-timer's plan turns this sequence into an hour-by-hour eclipse-day timeline.
Common Questions
How rare is a total solar eclipse in one place?
On global average, totality revisits any given spot about once every 375 years, because the shadow track is only a couple of hundred kilometres wide and lands somewhere different each time. Sydney's 2028 totality is its first since 1857.
What is the difference between magnitude and obscuration?
Magnitude measures how much of the Sun's diameter is covered; obscuration measures how much of its area is hidden. Totality requires magnitude 1.0 or more - Sydney's 2028 value is 1.025, while Melbourne's 0.844 means a partial eclipse with glasses required throughout.
What is the saros cycle?
A period of about 18 years and 11 days after which nearly identical eclipse geometry repeats. The 2028 eclipse belongs to saros 146; each successive member occurs a third of the way around the globe because of the extra eight hours in the cycle.
When is the next total eclipse in Australia after 2028?
On 25 November 2030, when totality crosses South Australia and New South Wales - the second event in an unusually rich Australian run that continues into the late 2030s.
How long can totality last?
The theoretical maximum is about seven and a half minutes. The 2028 eclipse peaks at just over five minutes near the point of greatest eclipse over remote north-western Australia; Bourke's 4 minutes 6 seconds is the longest city-centre duration in this guide.