How far away can you see the northern lights?

SWPC repeats the number on its tips page with a condition attached: a vantage point such as a hilltop with an unobstructed view toward the north. The same page warns that its storm maps show where aurora may be seen from, not where the aurora itself is, because it can often be observed hundreds of kilometers equatorward of the actual display.

So there are two distances to keep apart. One is how far the oval's edge sits from you. The other is how far away bright aurora can be and still clear your horizon. When the first is smaller than the second, you are in range.

SWPC puts the aurora at typically 80 to 500 km above the surface. Light from that height clears the curve of the Earth from a long way off. On a smooth sphere, aurora at height h sits exactly on the horizon for an observer at ground distance R times arccos(R / (R + h)), with R the mean Earth radius of 6371 km.

At 100 km the geometric limit is already over 1,100 km. The upper parts of tall rays reach farther still. In practice haze, terrain and the faintness of light near the horizon cut the range, which is why SWPC ties its 1000 km figure to bright aurora and good conditions.

Glow combines SWPC's two rules of thumb. The oval's equatorward edge sits near 66 degrees geomagnetic at Kp 0 and moves 2 degrees south per Kp step. One degree of latitude is 111.2 km. The table gives the distance north to that edge at Kp 0, 3 and 5, and flags where it falls within 1000 km.

Duluth sits 1,180 km south of the Kp 0 edge. At Kp 5 the edge moves to 56 degrees, which puts it within the 1000 km range for Duluth and the other lower 48 towns. Fairbanks is only 40 km from the Kp 0 edge, so even at Kp 0 the average edge sits far inside the 1000 km range.

Each Kp step moves the edge 2 degrees, about 222 km, closer to the lower 48. Seattle sits 1,440 km south of the Kp 0 edge and Buffalo 1,550 km. Subtract 222 km per step to see how far off the edge is on a given night.

The rule of thumb smooths the oval into a ring. On a real night it bulges and thins. The distance tool measures from your coordinates to the nearest live OVATION cell at 10, 30 and 50 percent, with a compass bearing, and compares it with SWPC's 1000 km.

• Face the bearing the tool gives, usually north, and find a low, open horizon in that direction.
• Use the southern edge tool to see how far south the oval reaches along each longitude.
• Expect aurora near the limit to sit low on the horizon.

Distance is only one input. Darkness and cloud still decide whether anything is visible. The chance tool sets the cell over you beside those two.