# Retrograde Periods

`POST https://api.astroway.info/v1/retrograde-periods`

Retrograde periods of the planets in `planetIds` (default Mercury) within a date range: the two stations and their longitudes, plus `shadowStart` (the planet first reaches the longitude of its direct station) and `shadowEnd` (it regains the longitude of its retrograde station).

Group: Calendar & Cycles. Auth: `X-Api-Key` header. Operation ID: `retrograde-periods`.

## Parameters

| Name | In | Required | Type | Description |
|---|---|---|---|---|
| `fields` | query | no | string | Comma-separated dotted paths, relative to `data`, to keep in the response. A path is applied to every element of an array, so `planets.name` means the name of each planet. Paths that match nothing come back in `_fields_unmatched`; a list where nothing matches at all is a 400, because an empty object would be a confident wrong answer. |
| `precision` | query | no | integer | Round every non-integer number in the response to this many decimal places. Integers are left alone, because an id is not a measurement. Ecliptic longitudes ship with fourteen decimals by default; 2 is about a tenth of an arcminute. |

## Request body

| Field | Type | Required | Description |
|---|---|---|---|
| `startDate` | string | yes |  |
| `endDate` | string | yes |  |
| `planetIds` | integer[] | no |  |

```json
{
  "planetIds": [
    2
  ],
  "startDate": "2024-01-01",
  "endDate": "2024-12-31"
}
```

## Responses

| Status | Meaning |
|---|---|
| 200 | Successful calculation |
| 400 | Validation error |
| 401 | Missing or invalid API key |

### Success fields, under `data`

| Field | Type | Required | Description |
|---|---|---|---|
| `periods` | object[] | no |  |

## Example

```bash
curl -X POST https://api.astroway.info/v1/retrograde-periods \
  -H "X-Api-Key: aw_live_..." \
  -H "Content-Type: application/json" \
  -d '{ "planetIds": [ 2 ], "startDate": "2024-01-01", "endDate": "2024-12-31" }'
```

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