阅读下面这首诗。
                                                                                                 次北固山下
                                                                                    客路青山外，行舟绿水前。
                                                                                    潮平两岸阔，风正一帆悬。
                                                                                    海日生残夜，江春入旧年。
                                                                                    乡书何处达？归雁洛阳边。