{
  "$schema": "https://lumavik.org/api/v1/openapi.json",
  "endpoint": "https://lumavik.org/api/v1/darkness.json",
  "description": "Hours of astronomical darkness month by month at each five-degree band of geographic latitude, the usable aurora season those hours produce, and the exposure model the camera-settings tool runs.",
  "model": {
    "twilight_threshold_degrees": -18,
    "definition": "Astronomical twilight: the sun more than 18 degrees below the horizon. A fainter threshold would overstate the season, because a faint arc is not visible while the sky is still brightening.",
    "method": "Standard low-precision solar declination series, evaluated at the midpoint of each month.",
    "usable_month_definition": "A month counts as usable at two or more hours of astronomical darkness."
  },
  "counts": {
    "latitude_bands": 8,
    "months_per_band": 12
  },
  "provenance": "Computed from the standard low-precision solar declination series against astronomical twilight (sun 18 degrees below the horizon), the same function the dark-hours calendar on this site runs. The season windows are the editorial table from the page named in `source`.",
  "caveats": [
    "Darkness is computed from a low-precision solar declination series and is accurate to a few minutes of twilight time, which is far below what matters for planning a night. It ignores terrain, altitude, refraction near the horizon, artificial skyglow and cloud — and cloud is what actually ends most attempts.",
    "None of this predicts a specific night. It says what a location needs; whether the Sun supplies it is a separate question answered by NOAA SWPC in real time."
  ],
  "related": {
    "thresholds": "https://lumavik.org/api/v1/visibility.json",
    "cities": "https://lumavik.org/api/v1/cities.json",
    "human_page": "https://lumavik.org/skywatch/northern-lights-season/",
    "calculator": "https://lumavik.org/tools/dark-hours-calendar/"
  },
  "licence": {
    "name": "CC BY 4.0",
    "url": "https://creativecommons.org/licenses/by/4.0/",
    "attribution": "Lumavik — https://lumavik.org",
    "note": "Free to reuse, including commercially, with a link back to the endpoint or the page a figure came from."
  },
  "darkness_by_latitude": [
    {
      "latitude": 40,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 11.2
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.3
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 9.1
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 7.5
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 5.9
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 4.9
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 5.4
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 6.9
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 8.6
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 10
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 11.4
        }
      ],
      "usable_months": 12,
      "darkest_month": "December",
      "darkest_month_hours": 11.4
    },
    {
      "latitude": 45,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 11.4
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.4
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 8.9
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 6.9
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 4.8
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 3.3
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 4.1
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 6.2
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 8.3
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 9.9
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11.1
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 11.7
        }
      ],
      "usable_months": 12,
      "darkest_month": "December",
      "darkest_month_hours": 11.7
    },
    {
      "latitude": 50,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 11.6
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.4
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 8.6
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 6.2
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 3.1
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 1.4
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 5.1
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 7.8
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 9.8
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11.3
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 11.9
        }
      ],
      "usable_months": 10,
      "darkest_month": "December",
      "darkest_month_hours": 11.9
    },
    {
      "latitude": 55,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 11.8
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.3
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 8.2
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 5
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 0
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 0
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 3.5
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 7.2
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 9.7
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11.5
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 12.2
        }
      ],
      "usable_months": 9,
      "darkest_month": "December",
      "darkest_month_hours": 12.2
    },
    {
      "latitude": 60,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 12.1
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.3
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 7.6
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 3
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 0
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 0
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 0
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 6.3
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 9.5
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11.6
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 12.6
        }
      ],
      "usable_months": 8,
      "darkest_month": "December",
      "darkest_month_hours": 12.6
    },
    {
      "latitude": 65,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 12.3
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 10.1
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 6.7
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 0
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 0
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 0
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 0
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 4.9
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 9.1
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 11.8
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 13
        }
      ],
      "usable_months": 7,
      "darkest_month": "December",
      "darkest_month_hours": 13
    },
    {
      "latitude": 70,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 12.7
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 9.9
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 5.1
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 0
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 0
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 0
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 0
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 0.7
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 8.5
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 12
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 13.6
        }
      ],
      "usable_months": 6,
      "darkest_month": "December",
      "darkest_month_hours": 13.6
    },
    {
      "latitude": 75,
      "months": [
        {
          "month": 1,
          "month_name": "January",
          "dark_hours": 13.2
        },
        {
          "month": 2,
          "month_name": "February",
          "dark_hours": 9.3
        },
        {
          "month": 3,
          "month_name": "March",
          "dark_hours": 0
        },
        {
          "month": 4,
          "month_name": "April",
          "dark_hours": 0
        },
        {
          "month": 5,
          "month_name": "May",
          "dark_hours": 0
        },
        {
          "month": 6,
          "month_name": "June",
          "dark_hours": 0
        },
        {
          "month": 7,
          "month_name": "July",
          "dark_hours": 0
        },
        {
          "month": 8,
          "month_name": "August",
          "dark_hours": 0
        },
        {
          "month": 9,
          "month_name": "September",
          "dark_hours": 0
        },
        {
          "month": 10,
          "month_name": "October",
          "dark_hours": 7.4
        },
        {
          "month": 11,
          "month_name": "November",
          "dark_hours": 12.3
        },
        {
          "month": 12,
          "month_name": "December",
          "dark_hours": 14.4
        }
      ],
      "usable_months": 5,
      "darkest_month": "December",
      "darkest_month_hours": 14.4
    }
  ],
  "season_windows": [
    {
      "geographic_latitude": "50–55° N",
      "window": "Late August to early April",
      "note": "Dark nights return earlier, but stronger activity is usually needed farther from the oval",
      "source": "https://lumavik.org/skywatch/northern-lights-season/"
    },
    {
      "geographic_latitude": "55–60° N",
      "window": "Late August to mid-April",
      "note": "A useful compromise between darkness and access to lower-latitude auroral activity",
      "source": "https://lumavik.org/skywatch/northern-lights-season/"
    },
    {
      "geographic_latitude": "60–65° N",
      "window": "Late August to mid-April",
      "note": "Long winter nights; twilight begins to limit late spring viewing",
      "source": "https://lumavik.org/skywatch/northern-lights-season/"
    },
    {
      "geographic_latitude": "65–70° N",
      "window": "Early September to early April",
      "note": "The summer gap is longer; the auroral oval is often closer overhead",
      "source": "https://lumavik.org/skywatch/northern-lights-season/"
    },
    {
      "geographic_latitude": "70–75° N",
      "window": "Early September to late March",
      "note": "Polar twilight compresses the season despite very long winter darkness",
      "source": "https://lumavik.org/skywatch/northern-lights-season/"
    }
  ],
  "camera_model": {
    "baseline_at_f2_8": [
      {
        "display": "faint",
        "label": "Faint, slow arc",
        "shutter_seconds": 10,
        "iso": 3200
      },
      {
        "display": "active",
        "label": "Active band with folds",
        "shutter_seconds": 4,
        "iso": 3200
      },
      {
        "display": "bright",
        "label": "Bright, moving curtain",
        "shutter_seconds": 2,
        "iso": 1600
      },
      {
        "display": "corona",
        "label": "Overhead corona",
        "shutter_seconds": 1,
        "iso": 1600
      }
    ],
    "moon_stops_recovered": {
      "none": 0,
      "quarter": 0.5,
      "half": 1,
      "full": 1.7
    },
    "star_trailing_rule": "500 / focal length in mm, in seconds — binding only on a faint, slow arc",
    "principle": "Shutter speed is set by how fast the display is moving, not by how dark the sky is. A brighter, faster aurora wants a shorter shutter and pays for it in ISO.",
    "source": "https://lumavik.org/tools/aurora-camera-settings/"
  }
}