How to enlarge and/or move a circular selection

I am trying to figure out if it is possible to create a circular selection and move it --the selection outline/marching ants, not the image-- so that I can match it onto an image which has a circular object.
I have tried many times to create a circular selection, using Shift to keep it exactly circular, but I can never manage to pick a starting point that makes the circle lay directly on top of the circular object in the image.
Hugely frustrating.
I have used some other image editors in the past (forget which one(s) allow it) which allow you to both (a) create a selection box/circle and move that selection thing (layer? marching ants) but without it pulling the picture along. That way I could make sure I got it centered over the circle object in the picture. and (b) allow to grow or enlarge the circular selection so the whole thing grows (radially- gets larger from the center).
I know that Acorn allows to add to a selection, but adding or deleting a ‘chunk’ of pixels to a improperly sized circle is maddening, as the result is no longer circular.

The test case here is an image of a circular object which has a bunch of artsy fading shadow junk underneath it. I am trying to get rid of the shadow below (and which also touches the edges) without having to resort to 1000% magnification and tiny eraser/pencil work…

Maybe its just not possible and I need to use a different graphic tool…
Acorn 6.6.4 OS X 10.12.6 Sierra older iMac.

After drawing…

Make sure the Marquee tool is selected to move the outline of the selection.

If the Arrow tool is selected the part of the image contained by the selection will be moved.

During drawing…

Hold space.

I suppose you could add an area to an existing selection - even some of the already selected area - to let you use Space on an already drawn selection? Not tested.

See the docs Acorn: Selections

Moving Selections

To move a rectangle or elliptical selection you are actively drawing, hold down on the space bar while dragging. Let go of the space bar to continue adjusting the size and shape of your selection.

Once you have made a selection, you can move it around with the arrow keys or move tool. In addition to this, you can hold down the ⌘ (command) key and Acorn will first cut out the selection and then move it around your image. Letting go of ⌘ will then place your floating selection.

You can hold down ⌘ and ⌥ (option) when clicking and dragging (or while using the arrow keys), and Acorn will continually copy and place your selection each time you release your click. Here is a video of it in action, using a screenshot of Acorn’s preferences window: Command Option Move in Acorn.

gingerbeardman has the right way to do it. Holding space while creating the selection will help you move it around (this is a standard shortcut in most major drawing apps, so you might find it implemented elsewhere).

You can also use the oval tool to draw exactly where you want your selection, and then use the Select ▸ Make Selection from Layer menu item.