Dorset is a county on the south coast of England with an area of 2653km² (1024 square miles), with a longer west-east axis of 89km (55 miles) and the north-south axis is 50 km (31 miles).

To the west is South Devon (VC3), although there is a finger of land there that although VC9 it lies partly in Devon and Somerset.  To the north-west is South Somerset (VC5) with a tiny bit of North Somerset (VC6) to the north, and this 'untidy' border has parts of VC5 within the Dorset border.  Wiltshire (VC8) lies to the north-east, and to the east lies South Hampshire (VC11), part of which lies within the current Dorset border following the boundary changes in 1974.  There is a considerable variety in the underlying geology of Dorset which contributes to the diverse landscape.  Roughly half the county is chalk uplands running from north-east to south-west, including Cranborne Chase and the Dorset Downs, with a narrow band running south-west to south-east that make up the Purbeck Hills.  To the west and north-west lie Jurassic clay deposits interspersed with limestones, and include the Blackmore Vale and Marshwood, with fertile vales supporting dairy farming, and a few small woods.  In the far west are rolling hills, and include the highest points in the county, with Lewesdon Hill at 279m (915') and Pilsdon Pen at 277m (909').

  Along the south coast lie a mix of calcareous clays and shales including the renowned Jurassic coast, a World Heritage site, the 29km of pebbles of the Chesil Beach enclosing the 16km Fleet lagoon, to the much quarried limestone of the Isle of Portland and Purbeck coast.  There are occasional small and isolated upland heath areas such as Lambert's Castle and Black Down near Portesham.  The remaining quarter of the county in the south-east comprises of the sandy heathland of the Poole basin, a triangular area extending from the New Forest west to Dorchester, north to Cranborne and south to Studland.  Much has been covered with coniferous forest, but large isolated fragments of lowland heath and bogs remain, and many have SSSI status.  There are three main river systems which all flow into the sea in a south-easterly direction.  The Frome and Piddle are chalk streams and drain into Poole Harbour, one of the largest natural harbours in the world.  The Stour that rises in Wiltshire to the north has its origins in clay soil, and drains into Christchurch harbour.  Close by, the River Avon which flows mainly through Wiltshire and Hampshire, also ends its journey at Christchurch.

The climate of Dorset is influenced by its location and local topography which has profound effects both on rainfall and temperature, with resulting diverse micro-climates.  It is on the transitional zone between the wetter maritime west and the drier more continental east.  The coastal areas are affected by the sea, being warmer during the winter but cooler during the summer.  The mean annual rainfall can be as low as the low 600mm's on Portland, to well over double that in the rain-shadow in the vales north of the Cerne ridge.  There can be large differences in temperature and rainfall between the higher ground and the nearby valleys.  This affects the vegetation and consequently the moth species distribution, providing an extremely interesting and diverse habitat and fauna.