Functions to convert between WGS84 and bing mercator

BingConvert.cs

using System;

public class BingConvert
{
    public static double[] WGS84toGoogleBing(double lon, double lat)
    {
        double x = lon * 20037508.34 / 180;
        double y = Math.Log(Math.Tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);
        y = y * 20037508.34 / 180;
        return new double[] { x, y };
    }

    public static double[] GoogleBingtoWGS84Mercator(double x, double y)
    {
        double lon = (x / 20037508.34) * 180;
        double lat = (y / 20037508.34) * 180;

        lat = 180 / Math.PI * (2 * Math.Atan(Math.Exp(lat * Math.PI / 180)) - Math.PI / 2);
        return new double[] { lon, lat };
    }
}

Hello. Excuse me, can you say, what is 20037508.34 here?