'there it is , ' my mother says , and what she means is that the dot we 've been nearing for weeks , the one that 's been growing into a larger dot with two smaller dots circling it , has now become even larger than that , growing from a dot to a disc , shining back the light from its sun , until you can see the blue of its oceans , the green of its forests , the white of its polar caps , a circle of colour against the black beyond .