"standard ruler" meaning in English

{
  "forms": [
    {
      "form": "standard rulers",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "standard ruler (plural standard rulers)",
      "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",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Astronomy",
          "orig": "en:Astronomy",
          "parents": [
            "Sciences",
            "Space",
            "All topics",
            "Nature",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Any astronomical object of known diameter whose distance can then be calculated from the angle it subtends"
      ],
      "id": "en-standard_ruler-en-noun-HcDx-ffE",
      "links": [
        [
          "astronomy",
          "astronomy"
        ],
        [
          "diameter",
          "diameter"
        ],
        [
          "distance",
          "distance"
        ],
        [
          "angle",
          "angle"
        ]
      ],
      "raw_glosses": [
        "(astronomy) Any astronomical object of known diameter whose distance can then be calculated from the angle it subtends"
      ],
      "related": [
        {
          "word": "standard candle"
        }
      ],
      "topics": [
        "astronomy",
        "natural-sciences"
      ],
      "wikipedia": [
        "standard ruler"
      ]
    }
  ],
  "word": "standard ruler"
}

{
  "forms": [
    {
      "form": "standard rulers",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "standard ruler (plural standard rulers)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "standard candle"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Astronomy"
      ],
      "glosses": [
        "Any astronomical object of known diameter whose distance can then be calculated from the angle it subtends"
      ],
      "links": [
        [
          "astronomy",
          "astronomy"
        ],
        [
          "diameter",
          "diameter"
        ],
        [
          "distance",
          "distance"
        ],
        [
          "angle",
          "angle"
        ]
      ],
      "raw_glosses": [
        "(astronomy) Any astronomical object of known diameter whose distance can then be calculated from the angle it subtends"
      ],
      "topics": [
        "astronomy",
        "natural-sciences"
      ],
      "wikipedia": [
        "standard ruler"
      ]
    }
  ],
  "word": "standard ruler"
}