The dotted line from the earth to the moon represents your line of sight when looking at the moon. The large moon image shows what you would see at that point in the cycle. For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight.
Where can I find some general guideline for welding ventilation? For processes such as welding, brazing, soldering, and torch cutting, the primary purpose of ventilation is to remove air contaminants from the worker's breathing zone. Different ventilation strategies may be needed in each case to remove air contaminants from the welder's breathing zone. General guidelines have been published in CSA W117.2 Safety in Welding, Cutting, and Allied Processes, and ANSI Z49.1 Standard Safety in Welding and Cutting.
The variety of shielding gases used in arc welding can be a confusing topic. However, the use of an external shielding gas is a necessary component for some processes, as it is a requirement for surrounding and protecting the arc and molten weld metal from contamination by the atmosphere. Shielding gas can also increase the smoothness of the arc and operator appeal.