Mark two points onto the meridian. We shall call them X and Y.

When we design your Interior Sundial, we will calculate the distance that these have to be from S.

As you mark these onto the ceiling, don't forget the small correction distance.

We now need two lines to run across the ceiling, crossing the meridian at X and Y.

To get these in the right place, mark some more points either side of the meridian.

You could do this by drawing two arcs centred on X and Y.
Fix one end of a piece of string to X. We will calculate the length of the string for you. Move the other end of the string to make a mark on the ceiling.

Repeat this for an arc centred on Y.

You might like to check this afterwards with a steel rule because string can stretch a bit.

If you do this again on the other side of the meridian, you can then draw one of the lines by joining these two points.

You'll need to repeat this all over again to get the other line.

The line that crosses the meridian at Y is a construction line and, like the meridian, you will probably want to remove it at the end.
It forms no real part of the finished design.

However, The line that crosses the meridian at X is different.
It is called the Equinox Line.
On the Days of the Equinox (March 21st, September 22nd) the sunspot will pass along the Equinox Line.

In Summer the sunspot will be closer to the window. In Winter it will be further away from the window.

This means that when you come to label the hourlines, you could use numbers to show Daylight Saving Time (or BST) near the window and Standard Time (or GMT) on the far side of the Equinox Line.

Never climb up without using a ladder or steps that are adequate for the purpose.
Always make sure that the ladder is firmly planted on level ground and fixed into position.

On the Marking Out page we looked at how to mark a meridian on the ceiling.

The point S at the Southern end of the meridian is vertically above the mirror.

This probably puts S somewhere inside the brickwork.

And yet we are going to have to mark some points on the meridian, measuring from the point S.

One way to do this is to put a mark on the meridian near to S. Then use a thread and a steel rule to measure the distance from S to this point.

To find the position of the hourlines, mark a point on the Equinox Line and on the construction line.

We will calculate the position of these points for you. Usually these will be given as a distance to the point from X or Y.

For example, here are the marks for the 11 o'clock hourline.

Then, to draw in each line, just join the dots.

As with the other lines, one way to do this is to stretch out a piece of string that has been rubbed with chalk. Then give the string a flick.

Some of the hourlines might not cross both the Equinox Line and the construction line.

To locate both ends of one of these lines we must use a different method.

We can give the position of any point in the same way that we used on page Marking Out 5.
Stretch out a piece of string. We will calculate the length for you.

Place one end on the reference point and use the other end to mark an arc on the ceiling. Then repeat this for the other reference point. The length of the piece of string will probably be different each time.

The end of the hourline will be at the place where the arcs cross.

It's a good idea to use a steel rule to check that the string didn't stretch a little while you were doing this.