"astronomical twilight" meaning in English

{
  "forms": [
    {
      "form": "astronomical twilights",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "astronomical twilight (countable and uncountable, plural astronomical twilights)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
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        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Times of day",
          "orig": "en:Times of day",
          "parents": [
            "Periodic occurrences",
            "Timekeeping",
            "Time",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The period in which the sun is between 12° and 18° below the horizon in the morning and evening."
      ],
      "id": "en-astronomical_twilight-en-noun-w5QPdZtg",
      "tags": [
        "countable",
        "uncountable"
      ],
      "translations": [
        {
          "code": "nl",
          "lang": "Dutch",
          "sense": "period of partial darkness",
          "tags": [
            "feminine"
          ],
          "word": "astronomische schemering"
        },
        {
          "code": "fi",
          "lang": "Finnish",
          "sense": "period of partial darkness",
          "word": "astronominen hämärä"
        },
        {
          "code": "is",
          "lang": "Icelandic",
          "sense": "period of partial darkness",
          "tags": [
            "neuter"
          ],
          "word": "stjörnurökkur"
        }
      ]
    }
  ],
  "word": "astronomical twilight"
}

{
  "forms": [
    {
      "form": "astronomical twilights",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
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      },
      "expansion": "astronomical twilight (countable and uncountable, plural astronomical twilights)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
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        "en:Times of day"
      ],
      "glosses": [
        "The period in which the sun is between 12° and 18° below the horizon in the morning and evening."
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "translations": [
    {
      "code": "nl",
      "lang": "Dutch",
      "sense": "period of partial darkness",
      "tags": [
        "feminine"
      ],
      "word": "astronomische schemering"
    },
    {
      "code": "fi",
      "lang": "Finnish",
      "sense": "period of partial darkness",
      "word": "astronominen hämärä"
    },
    {
      "code": "is",
      "lang": "Icelandic",
      "sense": "period of partial darkness",
      "tags": [
        "neuter"
      ],
      "word": "stjörnurökkur"
    }
  ],
  "word": "astronomical twilight"
}