# Observer location

To calculate local horizontal coordinates (topocentric) for an observer located at an arbitrary location on (or above) the Earth's surface, include a suitable value for the `obs` query parameter:

{% code lineNumbers="true" %}

```javascript
 axios({
	"method": "GET",
	"url": "https://api.radiantdrift.com/body-position/DATE_TIME_FROM",
 	"params": {
		"body": "[BODIES]",
		"obs": "LAT,LNG,HEIGHT"
	},
})
```

{% endcode %}

{% code overflow="wrap" lineNumbers="true" %}

```shell
curl "https://api.radiantdrift.com/body-position/DATE_TIME_FROM/?obs=LAT,LNG,HEIGHT&body=BODIES"
```

{% endcode %}

`LAT` is the observer latitude in decimal degrees and `LNG` is the longitude (negative west). HEIGHT is the observer elevation above mean sea level, given in meters. If no elevation value is available, use zero (0m).

**Example**

The following query calculates the position of the Moon for March 21 2025 at 13:04:30 UTC as observed in Boulder, Colorado, USA:

{% code overflow="wrap" fullWidth="false" %}

```
GET https://api.radiantdrift.com/body-position/2025-03-21T13:04:30Z/?obs=40.009728,-105.237709,1612&body=moon
```

{% endcode %}

A `localHorizontal` object is included in the response, giving true and apparent azimuth/altitude coordinates for the observer, together with parallax and the effect of estimated refraction.

```json
{
  "query": {
    "name": "body-position",
    "start-date": "2025-03-21T13:04:30.000Z",
    "obs": {
      "lat": 40.009728,
      "lng": -105.237709,
      "height": 1612
    },
    "alg": "standard",
    "high-accuracy": false,
    "body": [
      "moon"
    ],
    "ra-decl": null,
    "extras": [],
    "interval": 60
  },
  "response": {
    "2025-03-21T13:04:30.000Z": {
      "moon": {
        "dateTD": "2025-03-21T13:05:39.073Z",
        "jde": 2460756.04559112,
        "eclipticCoordinates": {
          "λ": 260.52729877,
          "β": -5.25429849
        },
        "geocentricSphericalCoordinates": {
          "λ": 260.52729877,
          "β": -5.25429849,
          "Δ": 398136.53563773
        },
        "apparentCoordinates": {
          "ra": -100.73114387,
          "decl": -28.34079256
        },
        "apparentLongitude": 260.52750373,
        "obliquityOfEcliptic": 23.43866925,
        "radiusVector": 13135.97563773,
        "nutationInLongitude": 0.00020496,
        "nutationInObliquity": 0.00265718,
        "equatorialHorizontalParallax": 0.91791617,
        "meanElongation": 267.39595465,
        "meanAnomalySun": 75.93822925,
        "meanAnomalyMoon": 237.2096644,
        "argOfLatitude": 269.42006603,
        "localHorizontal": {
          "true": {
            "azimuth": 190.28208172,
            "altitude": 20.04241573
          },
          "apparent": {
            "azimuth": 190.28208172,
            "altitude": 20.08140602
          },
          "parallax": {
            "azimuth": 0,
            "altitude": 0.85634116
          },
          "refraction": 0.0389903
        }
      }
    }
  }
}
```

The effect of atmospheric refraction is generally to increase the altitude of the body for the observer, as seen in the example above. The effect is more pronounced near the horizon.