=Aurora borealis.= The Northern Lights. A glowing light effect which takes place in the Arctic Circle. It can often be seen from our Northern States and sometimes from the Southern States. The same kind of lights appear in the Antarctic Circle. These Southern Lights are called Aurora Australis. =Axes.= Plural of axis. =Axis.= An imaginary line on which a body turns. =Axis of rotation.= An imaginary line round which anything turns or spins. =Axle.= A wood or metal rod on which one or more wheels turn, or it may turn with the wheel or wheels. =Badge of merit.= A badge awarded by the organization of Boy Scouts to members who can show a certain amount of skill in doing certain things. See Appendix A. =Band.= (1) A flat strip of any material, or (2) such a strip whose ends are joined together. =Beam.= Rays of light which are parallel. See Ray. =Bearing.= (1) A piece of metal, or a bit of agate or other jewel, on which a pivot, spindle or shaft turns. (2) A direction as found by the Sun or stars, or by the compass, sextant or other means. =Body.= Any separate and distinct amount of matter when held together. The Sun and Moon are heavenly bodies. =Degree.= (1) The 360th part of a circle. (2) Indicated by this sign °. =Device.= (1) An apparatus or instrument or any part thereof. Any arrangement for producing a given result. =Diagram.= A sort of shorthand picture of an apparatus or part of an apparatus. It is used to show in the simplest and clearest manner the construction of an apparatus. =Diameter.= See Appendix B. =Direct line.= A straight line. =Disk.= (1) A circular plane or flat surface. Since the Sun, Moon and planets seem to be flat their surfaces are called disks. (2) Flat and circular like a coin. =Dog days.= So called from the fact that Sirius, the Dog Star, rises at the same time as the Sun. =Ecliptic.= See Appendix B. =Elastic.= (1) A body that will stretch and return to its original size. (2) A thin piece of rubber. =Ellipse.= See Appendix B. =Equator.= See Appendix B. =Equatorial.= Having to do with or affected by the equator. =Equatorial telescope.= A telescope so mounted that its principal axis is parallel with the axis of the earth and moved by clockwork so that it will follow a star. =Evening Star.= A planet which can be seen in the west just after the sun sets. =Fixed.= (1) Fastened securely. (2) The stars are called fixed because they never seem to change their positions.